Ubuntu 8.04 (32bit), using mostly gedit and 2 or 3 terminal windows does the trick for me. Have tried Netbeans and Aptana/RadRails but I am not really into the kind of automation they provide, so a basic editor with some minimal support (code highlight and integrated file browser) is more than enough for me.
For development, Mac OS X For production, Fedora Core and Cent OS
Using Win XP for development and Linux for deployment, but I plan on moving to a Mac pretty soon.
OSX at: Work Personal MacBook and iMac
Leopard and TextMate on all of them.
I hope I never have to develop in Windows again.
Mac OS X on my development laptop, Ubuntu Linux on my home server. I keep each updated to the latest version (right now that's Leopard and Hardy Heron). I just switched from Mongrel to Phusion Passenger on the server.