The best site by far is
http://www.emulation.net for all of your mac emulation needs and info. Richard Bannister has done an outstanding job of maintaining this site - he's been running it for as long as I can remember, and it's one of my favourite places to visit...
One thing I promised myself when I bought a powerful Mac last year was to put aside a couple of hundred megabytes solely for emulation, and retro games. I mean, after all - what's a half-a-gig of disk space between friends nowadays, huh?
So - now, I have (under emulation, and, in some cases, carbonised for OSX) these systems on my Cube...
An Amiga
A Sinclair Spectrum 48K
A Sinclair ZX81
A Commodore 64/128
An Atari 1040ST
An Apple IIGS (Karateka never looked so good!)
A Playstation (just insert the disk - and away you go!)
A Nintendo 64
MacMame (The World's best Arcade emulator! At least5000 games!)
And even a Pentium under Windows 98SE (Does Virtual PC 5 count?)
Yes, yes. I know I'm a sad bastard. And I probably can't let go of the past - but it is a great way of reliving old memories.
So, whenever I want to return to those halcyon days of "Peeks" and "Pokes", of "Gosubs" and "Gotos", of Machine Code and Pascal, then all I have to do is launch an emulator (not forgetting of course to run a nice 1980's Compilation CD using iTunes in the background) whilst chanting - "There's no place like home - there's no place like home...".
Kinda hard to do when you're using a nice new LCD display, though.