68k Developers
Here are links to the websites of other developers in the 68k community. If you'd like to be included here, send me an email!

68k Open Source
There are many open source 68k projects that can provide helpful examples of actual working code. Here are a a few of them.
MacGZip - 68k and PPC
MacSSH - 68k and PPC
Classilla - Web browser based on Mozilla. Runs on OS 8.6 - 10.3.9.
Ingemar's Corner - A collection of code demos in both C and Pascal.
MacPaint and QuickDraw - The original 68k assembly source code for these programs has been made available for non-commercial use.

System Software
Apple's Software Archive - You can download System 6 and 7 here, as well as ResEdit and various other tools.

Mini vMac - 68k emulator, better maintained than Basilisk II
Basilisk II - 68k emulator
SheepShaver - PPC emulator

Transmit - Excellent FTP client for Mac OS 9.

Programming Languages
Caml Light - Open source.
Chipmunk Basic - Version on site is for Mac OS 9. Versions exist that go as far back as System 6.
Icon/ProIcon - Open source.
MacGofer - Open source.
MacMETH - A Modula-2 compiler. Open source.
MacPerl - Open source, supports System 7.5.5 - Mac OS X.
MacPython - Open source, 68k through OS 9.
Open Prolog - System 7.5.5 - OS 9.
PowerLisp - Open source, PPC and 68k.
UCBLogo - Open source.