Your Favorite Resolution Systems

[LibraryOgre]

One simple one for combat and other situations where success is measured as the combination of many actions each with its own resolution check, basically either d20 or d100 plus simple and not often changing mods vs fairly straightforward TN.

One complex system, preferably bell curve based, that strongly reflected character(s) skill and lots of fiddly modifiers, for times when a whole host of player decisions is ending up being abstracted into a single roll for success. Also with degrees of failure and degrees of success.

Hackmaster does something like this in skill checks

Unopposed tests are d100% under your skill percentage; average tasks have a -40% on the roll, while difficult tasks are straight roll (there's more modifiers, but those give you some idea).
Opposed tests are d100+skill modifier v. d100+skill modifier of the opponent.

For example, if someone is sneaking past a relatively inattentive guard, the GM might call for an unopposed test, or they might call for an opposed test. Making a basket or asking for information is likely to be unopposed; interrogation or picking pockets is more likely to be opposed.