Scumming techniques are certain repeatable actions or tactics providing players with unusual in-game benefits, usually exploiting a bug or simply quirks of the game design that might not have been intended to be used this way, or to this extent. They vary from commonly employed practices like the ID stairhop to the abuse of piety overflow, which is frequently looked down upon since it involves heavily exploiting a programming oversight.
Note that the amount of scorn you will receive for using these strategies is inversely proportional to your newness to the game, the desperateness and difficulty of your situation in game when you decided to use one of these techniques, and directly proportional to how much you tend to brag about your successes. As a newbie, don't shy away from using these techniques in order to make it easier for you to gather more experience and become more skilled in the game in general. Many of these are perfectly fine elements of the game, and only those who are bored otherwise will set themselves the restriction not to use them (and sometimes fail to understand that not everyone shares their viewpoint). The most important rule of all is to have fun.
Many scumming tactics present in ADOM v. 1.1.1, were fixed during v. 1.2.0, either due to RFEs focusing on balancing the game or because they stem from bugs that would be fixed during development anyway.
Save scumming - This refers to using backups of the savegames to continue playing a particular character after he has died. This is universally regarded as not in keeping with the tradition of roguelike games, which expect you to succeed without dying at all, and thus a victory achieved after save scumming is not regarded as a true victory. Many will find this method necessary in order to reduce frustration during the very steep learning curve.
Gremlin bomb - creating lots of easy to kill gremlins and using them to get easy loot, be it by killing them, or by pickpocketing and then killing them, or having a pet kill them for you (and pickpocketing them for good measure). It's not workable for XP since gremlins give few to begin with, and though they grow more experienced, XP gain is reduced the more often you kill a monster. Note that having items and the knowledge of how to use them are different things. 1.2.0 status: Summoned monsters no longer drop loot nor can be pickpocketed, making gremlin bombing a useless tactic.
ID stairhop - continuously going up and down stairs between two certain ID levels. One could also explore them instead, but stairhopping exposes a maximum of floor tiles per turn. Loot created inside the room is picked up, and accumulates over time. Requires some patience, but is very safe in general, much safer than trying to find the respective loot in-game. Refraining from this technique - or, indeed, from entering the ID at all - will make the early game considerably more challenging. 1.2.0 status: Items are no longer generated on the floor on stair rooms in the ID.
Piety overflow - abusing integer overflow to turn a huge negative piety into a huge positive and milk your god for goodies. Nowadays this is considered to be a quite cheap and unnatural bug exploit by most ADOM players, but some legit Hall of Fame entries used it. It should be kept in mind that if there was an opportunity in game to achieve integer overflow of experience, it would most certainly defeat the purpose of almost the entire game; this "only" defeats the purpose of the sub-game of dealing with divine forces and provides virtually unlimited prayers, with all their benefits. 1.2.0 status: Several exploitable problems stemming from integer overflow have been fixed.
Dragon gold doubling - Dragons which pick up gold drop double the amount in addition to their own gold upon death. Currently there is quite a debate if this is a bug or an intentional feature. Due to its exponential nature, this can easily be used to create truly game-breaking amounts of gold, which are commonly spent on getting piety for precrownings or to pay for Garth's training. There is a bit of risk involved since many types of dragons can destroy entire heaps of gold with their breath attacks, and dragons are relatively rare in the early game and cannot double more than once. Even if the player wants to avoid this effect, it's not always possible to keep dragons from picking up gold in tension rooms or vaults. 1.2.0 status: Dragons no longer replicate gold.
Stat lowering - intentional lowering of stats by controlled sickness and/or starvation to train stats beyond the common limit of certain methods, such as herbs. Requires some cautiousness. Is not considered to be heavily game-breaking, it only squeezes out a bit more extra stats from a very safe stat training method, and opens up the possibility of being ridiculed for dying of starvation in this manner (even sickness can be deadly, but players will usually have it removed before that happens). 1.2.0 status: Stat training uses base stats instead of current stats, limiting the effectiveness of these measures.
Wish engine - mostly considered to be to game breaking - once it gets going, the player has basically unlimited wishes at his disposal, which makes the rest of the game about as easy as it can be. The fact that 19 rings can be dipped at once, and a wish for potions of exchange usually returns three potions, contribute to the effectiveness of this method. Even 19 wishes, as gained from one successful dip, are more than enough to increase the character's power to game-breaking levels. To gain enough potions of exchange and a stack of 19 identical rings Gremlin bombs are frequently employed. 1.2.0 status: Dipping a stack of items in a potion of exchange now transforms each item individually -- making it harder to get such large stacks of the same item.
Ring of weakness - easily abused, combined with some potions of exchange and plenty of food and time almost all of your important stats can be increased to 99. Very game-breaking - the player can only die to severe cases of carelessness afterwards. 1.2.0 status: Same as stat lowering.
Self education - writing lots of scrolls of education and using them to raise/get some skills. Under certain not really known conditions scrolls can be created without any stat drains, but losing a few points of mana is not unexpected, even with a blessed scroll and writing set. Not that scummy since it does require magical writing sets, rather rare tools, and getting really good skills are not easy to get.
Grinding - usually staying on cavernous levels with high monster generation to kill lots of monsters gaining xp, items and weapon skill marks in process. Not very scummy since it is not entirely risk- and hassle-free, depending on the location where you perform it. More dangerous locations will yield greater returns, so for a challenge choose a place where you will be faced with monsters that actually give a fight. Since you will naturally meet monsters and kill them while pursuing your regular goals, such as the overarching quests or exploring dungeons, you often don't have to specifically grind at all.
Shop restocking - repeatedly buying out all of the items from a shop so that a shopkeeper will produce more items. Can be combined with kick robbing occasionally. Provides a challenge in that obviously you have to get the gold somewhere; of course, gold is not exactly an issue if you use other scumming techniques such as dragon gold doubling.
Kick robbing - opening a hole in a store wall that allows the PC to kick items out of a shop without enraging the shopkeeper. Most definitely not supposed to be possible, as it usually happens in plain sight of the shopkeeper, and pretty exploitish. 1.2.0 status: Fixed.
Milking - some monsters always, eventually, throw/shoot things at a character which they would not ordinarily drop if they were killed instantly. Allowing them to attack you will ensure you get these items. A common method to stock up on arrows or quarrels, or obtain an orcish spear as early in the game as possible.
Pickpocketing - used to gain more items from monsters, as after a certain point about 99% of the creatures you will encounter are weak enough that you can easily afford spending another turn on a Pick pockets attempt. Everything that weighs 10 stones or less can be pickpocketed from any applicable creature, and items pickpocketed will not in any way lower the amount of items you gain from killing - in other words, you will find easily double the (below 10s) items in total, and thus be much more powerful than another player who did not pickpocket at all by sheer availability of resources. There are no good reasons to not pickpocket other than it involves pressing additional buttons (at least two), while most small fry is usually killed in one or two hits. Due to the heavy increase in total loot, a habit some find worthy of picking up, and others can't stomach. The difference is considerable, but nothing which will decide over life and death; a non-pickpocketer will probably simply find himself grinding more often than the pickpocketer.
Banshee in animated forest. One of ways to scum for items is to bring banshee into animated forest. High rate of tree spawning combined with area instakill effect leaves some items rather soon. Although there is danger in banshee gaining some levels and becoming rather dangerous in melee too. 1.2.0 status: Plants are now immune to the banshee's wail.