Depends on where I'm working Windows with Netbeans :'( or Windows with MED TextEditor or Ubuntu with tuned Gnome :) and ever using Git
I use Ubuntu 8.10, but I keep thinking I'll snag a Mac mini this summer specifically for Rails and TextMate. I guess it's not just me!
I use RedHat Linux for a production server. I develop on Mac OS X Leopard with TextMate and GitX or Syncro SVN. I use Capistrano for my deployments and haven't any problems. I've also developed on Windows and Ubuntu, but OS X is my favorite.
Debian GNU/Linux with emacs, git, capistrano, mysql, memcached, etc. In production, same OS plus Apache with Phusion Passenger.
I like to use the same OS like on my VPS, this is about Linux. Telling the truth, I have the same working environment on my laptop, as I will be use on VPS. Few advantages here: 1) have no unexpectation behaviour by my application 2) familiar shell commands and other stuff and something else