I agree about Stella; the Atari 2600 is the only system left for TASing which I really like (and the ZX Spectrum, but I've yet to find a decent open-source emulator of it with save-state support :P).
Here are other emulators of systems that don't particularly interest me:
Atari++ (Atari 400 and 800, Atari 400XL, 800XL and 130XE, and Atari 5200)
http://www.xl-project.com/
It has GUI and savestate support.
blueMSX (SG-1000, SC-3000, SF-7000, ColecoVision, SVI318/328, MSX, MSX2, MSX2+ and turboR)
http://www.bluemsx.com/
GUI, savestate support and AVI capture.
ColEm (ColecoVision)
http://fms.komkon.org/ColEm/
GUI and savestate support.
MAME (Arcade)
http://www.mamedev.org/
It has a semi-decent GUI, AVI capture and input recording, but lacks proper savestate support for most of its supported games.
MESS (Every console/computer you can think of)
http://www.mess.org/
It has a semi-decent GUI, AVI capture and input recording, but lacks proper savestate support for most of its supported systems.
O2EM (Magnavox Odyssey²)
http://o2em.sourceforge.net/
Lacks both GUI and savestate support. Yeah, it sucks, but it's the only emulator besides MESS that can run the amazing K.C. Munchkin.
Potator (Watara Supervision)
http://sourceforge.net/projects/potator/
Lacks both GUI and savestate support.
ProSystem Emulator (Atari 7800)
https://home.comcast.net/~gscottstanton/
It has GUI and savestate support.
Raine (Arcade)
http://rainemu.swishparty.co.uk/
Another open-source arcade emulator. It has support for
these games. It also has support for Neo Geo games if it's compiled using makefile.neocd to build
neoraine.
Red Dragon (Virtual Boy)
http://rdragon.vr32.de/
GUI and savestate support.
These are of systems we already have rerecording support for, but these emulators may be better options than the currently available ones:
1964 (Nintendo 64)
http://sourceforge.net/projects/schibo/
An emulator a lot better than Mupen64 and the best one for Nintendo 64, but only for Windows. There is now source-code for the latest version. It has a GUI and savestate support.
Handy (Atari Lynx)
http://handy.sourceforge.net/
GUI and savestate support.
NeoPop (Neo Geo Pocket and Neo Geo Pocket Color)
http://neopop.emuxhaven.net/
GUI and savestate support.
Ootake (PC Engine)
http://www.ouma.jp/ootake/
Input recording, GUI and savestate support.
OSwan (WonderSwan)
http://sourceforge.jp/projects/oswan/
It has a GUI and savestate support.
EDIT: Also,
DOSBox is missing in the "Rerecording Tools but not yet approved for TASVideos" list (DOSBox v0.73 was released a couple of days ago, by the way).