Alright, finally a sticky with all the Simulators
(Note: Most of the sims mentioned here are hosted at GDAMania.net, which requires registration before downloading.)
DRUMMANIA/GUITAR FREAKS
(There are many lesser sims, but only the ones worth mentioning will be in the list)
DTXMania [Homepage] [Download]
The most commonly used simulator for DM/GF.
Supports both GDA and DTX files.
DTX files allow animation/background videos.
Allows grouping of song charts (bsc, adv, ext) to be grouped together using set.def files.
BandJam [Download]
An older sim, with larger Guitar display.
Does not support DTX files.
SessionStream [Homepage + Download]
Another sim that isn't used too much anymore.
Sims in development DigiBand -
Open Source. [Homepage + Download
New open-source simulator.
DMX [Homepage + Download]
Another sim that has been making lots of progress. Made by BillOB.
Worth trying out.
KEYBOARDMANIA DoReMi Mania [Homepage + Download]
The only main and good KeyboardMania sim. There is no need for any other.
You can put hook up Midi keyboards to it also.