Emulators

DOSBox (OS X) 0.74 FREE DOS emulator.
http://www.dosbox.com/information.php?page=0
Requirements: Mac OS X 10.3 or later.
DOSBox is a DOS-emulator that uses the SDL-library which makes DOSBox very easy to port to different platforms. DOSBox has already been ported to many different platforms, such as Windows, BeOS, Linux, MacOS X...DOSBox also emulates CPU:286/386 realmode/protected mode, Directory FileSystem/XMS/EMS, Tandy/Hercules/CGA/EGA/VGA/VESA graphics, a SoundBlaster/Gravis Ultra Sound card for excellent sound compatibility with older games... You can "re-live" the good old days with the help of DOSBox, it can run plenty of the old classics that don't run on your new computer!

E-UAE 0.8.29 FREE Version of the Amiga emulator (was (SDL)uae).
http://www.rcdrummond.net/uae/
Requirements: Mac OS X 10.2 or later. SDL Library
(SDL)uae is a new version of the popular Amiga emulator (UAE). This 'experimental' version of UAE is aimed at bringing all the cool features of WinUAE to non-Windows platforms. It incorporates the lastest CPU and custom-chip emulation from WinUAE, a bunch of bug-fixes, and I am gradually implementing for other platforms all those neat features that WinUAE offers. Linux, OS X, AmigaOS and BeOS are all currently supported with varying degress of functionality, although UAE should build and run on any Unix-like system.

MAME OSX 0.135 FREE Native OS X port of the popular MAME emulator.
http://mameosx.sourceforge.net/
Requirements: Mac OS X 10.4 or later.
MAME OS X is a native OS X port of the popular MAME emulator. It is designed to take advantage of all the latest OS X technologies, like Core Video and Core Image.

Mini vMac Full Edition 1.0 FREE Popular classic emulator.
http://devin.gotdns.com/downloads.html
Requirements: Mac OS X 10.3.9 or later.
Mini vMac Full Edition... I took the popular classic emulator and set it up for Mac (Universal). It includes System 1.1 with Finder 1.1 to System 7.0.1 with Finder 7.0.

Power64 (OS X) 4.9.5 FREE Commodore 64 emulator.
http://www.infinite-loop.at/Power64/index.html
Requirements: Mac OS X 10.1 or later.
Power64 is an emulation program that allows you to run Commodore 64 programs with full color and sound.

SheepShaver 2.3 FREE Emulator for Classic Mac OS environment.
Requirements: Mac OS X 10.2.8 or later.
SheepShaver is an Open Source PowerPC Mac OS run-time environment. That is, it enables you to run PowerPC Classic Mac OS software on your computer, even if you are using a different operating system. However, you still need a copy of Mac OS and a PowerMacintosh ROM image to use this program. SheepShaver is distributed under the terms of the GNU General Public License (GPL). Here's a page of instructions detailing how a SheepShaver user set it up on his Mac.

SIDPLAY (OS X) 4.1.3 FREE Commodore 64 music player.
http://www.sidmusic.org/sidplay/mac/
Mac PowerPC G4+, Mac OS X 10.5.0 or later.
SIDPLAY is a Commodore 64 music player. It emulates the sound chip and other internals of the Commodore 64 home computer to play back music that was originally written on it. The sound chip was called the "Sound Interface Device" or SID, and therefore this music is known as SID music. The SID chip is basically a three voice synthesizer on a chip. The unusual combination of digital and analogue circuitry is the reason for its distinctive sound.

Sixtyforce 0.9.5b1 FREE Play Nintendo 64 games on your Mac.
http://www.sixtyforce.com/
Requirements: Mac OS X 10.2 or later. Version for Mac OS 9 is available here.
Sixtyforce is an emulator that runs Nintendo 64 games. It does this by dynamically translating the code that a Nintendo 64 uses into something your Mac understands. Nearly every part of a Nintendo 64 has been painstakingly recreated entirely out of software to pull off such an amazing feat.