Activewire

Activewire is a USB I/O board with 16 programmable I/O pins that can interface to almost anything. This board has served me well. Visit my activewire page.

Arduino

Must have link, the arduino mac guide page.

Want to use XCode? The instructions on NICK VERWYMEREN page's worked fine for me.

Buying boards, parts and other links

Check Pololu, good selection of user-programmable microcontroller boards, modules and kits.

Capilano Computing offers great ECAD software for Mac users. They are the makers of DesignWorks Professional an easy-to-use electronic circuit schematic drawing tools. You can also find other great software in their web site.
Product Page

CadSoft is the maker of Eagle a schematic and layout program. Since Version 5.0 it runs native and no longer requires X11.
Product Page

Delcom focus on usb solutions and provide pre-programmed chips and OS X drivers for the Cypress EZ-USB series. They also have a nice USB starter kit - works great on OS X.
Company Page

Code Mercenaries are the makers of IO-Warrior a I/O board that comes in two models one with 32 generic I/O pins and one with 16 I/O pins.
Product Page