VMWare - From free PC emulators to enterprise-level server emulators, these guys are the big boys in the market right now
Virtual PC 2004 - One of Microsoft's emulators competing with VMWare is now free. VMWare has emulators that run on Linux, however and Microsoft (obviously) does not.
Virtual Server 2005 - Another of Microsoft's emulators competing with VMWare. This one is for setting up servers on an emulated computer. It's also free to download and use.
The original MAME site - Downloads for the console (non-graphical) version of MAME
MAME32 - MAME long ago switched from 16-bit to 32-bit, but MAME32 is simply MAME with a built-in graphical interface making it easier to use.
TI Calc.org - Site that features many programs and games for use with TI calculators. Calculator emulators can also be downloaded here.
http://www.viceteam.org/ VICE - A Commodore 64 emulator that also emulates a number of other similar systems
Hoxs64 - A newer Commodore 64 emulator that I've never tried, but looks promising.
JaC64 - Cool! It's a Java Commodore 64 Emulator that runs in your web browser! (warning, it takes a long time to start up)
SNES9x - One of the best SNES emulators (amoung many) that I've ever tried. Easy to use interface and reliable emulation
Zophar's Domain - Information on numerous emulators for various OS's
UAE - An Amiga emulator
ePSXe - A very good Playstation emulator
Project 64 - A Nintendo 64 emulator that is fairly up to date