C64 Emulators:
In order to play your favorite C64 games, you'll first need one of the following emulators.

Unlike emulators for other 8-bits, the C64 emulators require a lot of processing power because of the custom hardware, especially it's VIC-II graphics chip (play a game such as Pitstop II and see what I mean), which was used, and also the 2nd processor emulation required in the CBM-1541 diskdrive (it used a 6502). The list here is for DOS/Windows PC's, although in the cases of VICE & Frodo, the links will take you to the emulator's homepages (so you can get the link to the version you need)

MacOS users would be advised to check out www.emulation.net which offers information about emulators for many different platforms

For DOS:
CCS64 Version 1.09Homepage:http://ccs64.fatal-design.com
Sharware ReleaseRequirements:Pentium 60, 8mb RAM, DOS 5 (Will run under Windows 95 in a DOS box), VGA

The king of DOS based emulators. The only one which can flawlessly emulate complex raster effects, and can play the classic Pitstop II without any problems. Another bonus is the excellent TAP support, which is unable to run about 2 titles (although a few more in the Shareware version). The biggest problem is the inability to disable 1541 emulation for games which don't have a turbo loader, but this is negated by the ability to turbo-load single file programs (making the load almost instantaneous).

CCS64 Version 2 BetaHomepage:http://www.computerbrains.com
Shareware ReleaseRequirements:Pentium 60, 16mb RAM, DOS 5 (Can run in a Windows 95/98 DOS box), VGA

This public beta release is even better, fixing up the minor CPU emulation problem, and adding support for cartridges (with support for the legendary Action Replay Mk6, and even the Ocean Carts (I can't wait for someone to dump Battle Command :) )), alternate ROM sets, and the infamous Parallel cable hookups between the '64 and the 1541 (boosting performance as well). You can also run NTSC games, and the disk emulation has been upgraded to have continuous writes to a disk image (unlike 1.09 Registered). Finally, compared to 1.09, this has also had a huge speedup, with rumors of this being able to hit 100% speed on a high-end 486!

Personal C64 (DOS)Homepage:http://www.funet.fi/pub/cbm/crossplatform/emulator/msdos/pc64
Public DomainRequirements:386/25, 640k RAM, VGA, Mouse

Designed for 80486 systems, the incredible speed which this emulator runs at is its major advantage. The biggest problem with it though is the complete lack of processor level 1541 emulation (meaning that the majority of programs will not run at all), but

C64SHomepage: http://www.phs-edv.de
Shareware ReleaseRequirements:Fast 386, 640k RAM, VGA, DOS ?? (Will not run under Windows 95)

Although this emulator is able to draw some complex effects better, and has fastloader support, the demo will only run for 10 minutes, and it's very expensive. Not as graphically powerful as CCS64, but good enough if you have a (very) quick 486 or slow Pentium.

VICE (The Versatile Commodore Emulator)Homepage: http://www.cs.cmu.edu/~dsladic/vice/vice.html
Freeware [Under GPL]Requirements:Pentium, 8mb RAM, DOS 5, VGA

The second best emulator, and also, a multi-platform one, which started out life as X64 on UNIX, and now also is put together with a VIC-20 emulator, a PET emulator and the only C128 emulator in existance (use it to play the C128 games online). It can emulate the VIC-II to just below CCS64 levels, with minor glitching on Mayhem in Monsterland and Pitstop II (Two of the most complex C64 games ever made, in terms of graphic effects.) The new 1.1 release adds full 80 columm support in C128 mode (although annoying to use under DOS), and TAP support in the other emulations.

For Windows (3.X and 9X/NT):

CCS64 Version 2 Beta (Windows version)Homepage:http://www.computerbrains.com
Shareware ReleaseRequirements:Pentium 60, 16mb RAM, Windows 9X/NT, SVGA

With all the advantages of the DOS ports, this Windows version is very capable, although the display is limited to typical DirectX screen sizes (although 400 X 300 isn't a problem), although I've heard of severe audio problems for this port.

FrodoHomepage: http://www.http://www.uni-mainz.de/~bauec002/FRMain.html
FreewareRequirements:Pentium, Windows 95/NT

A stupid name for an emulator, but this one can do it's job quite well. It's only problem is that it lacks support for the PC64 file image format (i.e. P00/R00/U00/S00), but this is negated by the built in LYNX support (it treats them like disk images). It's also a ported emulator, with versions running on UNIX, Amiga, Mac and a few others. Frodo also has some complex disk emulation (Activision's Portal, from the C64 15 Pack will only run on this), but the highly accurate version [FrodoSC] is incredibly slow (Basically unusable on my P166).

Win64 Homepage:http://www.geocities.com/SiliconValley/Park/6558
SharewareRequirements:Pentium, Windows 95/NT

An average emulator, because the demo (or beta) version doesn't allow multiload games, and its very slow (even running DirectDraw fullscreen).  It comes with some games, and the full version is much better (and is bundled on a CD with lots of C64 stuff).

WinVICE 1.0Homepage: http://www.cs.cmu.edu/~dsladic/vice/vice.html
Freeware [Under GPL]Requirements:Pentium, SVGA, DirectX

An excellent port of VICE to Windows 9X/NT, this is finally up to date with the DOS and Unix ports, and as such, IMO, makes it the best of the current breed of Windows emulators. As well, the same updates from the DOS version apply here as well, and the support for 80 column C128 displays is very well implemented here too.

SID Sound Chip Emulators:
Powered by the 6581 SID sound chip, the C64 was also well known for its spectactular sound tracks. Now, it is possible to actually play these tunes as they sounded (i.e. not rendered in a conventional sound format), using these programs.

SIDPlayHomepage: http://www.geocities.com/SiliconValley/Lakes/5147/
FreewareAvailable for:MacOS, Win9X/NT, Unix, OS/2, DOS (old and unsupported)

The well ported SidPlay, is the most common program used, and in addition to its availability on multiple platforms, is basically the universal standard for playing these tunes on other machines. You can also grab the 'SIDPlug' plug-in for web browsers here as well.