OSX Leopard for my laptop (Textmate as well :p ). Servers are running Ubuntu and Debian
I develop on a MacBook running Leopard, and deploy to various Linux servers (ubuntu, gentoo, SuSE and Redhat).
I'm using TextMate here for my Rails development. I leave Eclipse for Java work (are you allowed to mention the J word here? :) ).
I've tried the rails support in Netbeans and Aptana Radrails but I found it too slow as I'm used to a plain text editor now.
@Housni Y: your question made me do some research... Microsoft claims that NTFS doesn't work well when you got multiple OSs. Thing is, I access and write to my Windows XP NTFSpartition from Kubuntu all the time. why should he be using FAT32?
Ubuntu. By far the best. OS X is pretty good, though I find some bits of OS X annoying.
I've used Ubuntu and Mac OS to develop RoR applications. For development, I like OS X and Textmate. Deployment is a whole other story though, with Linux being king.
For the last couple of years, it's been developing on OS X and deploying to Ubuntu servers. Haven't hit any showstopping glitches yet!
I am currently using Arch Linux, a lightweight Linux distribution with an excellent package manager.
Despite previous experience with Linux on the desktop, I tried for the longest to set up a comfortable Ruby development enviroment under Vista, only to fail repeatedly. I used tricks such as Cygwin, running Linux under Virtual PC and in VmWare, or running the development enviroment under Linux and accessing it remotely. In the end I got tired and installed Linux again. I also got frustrated by Microsoft's apparent ignorant stance toward power users. A command line which doesn't make me want to bang my head in the table? Hello! It's not asking for much..
Might consider OS X one day, but don't feel comfortable with the whole Mac package deal thing.
I might add that I actually like bits and pieces of Vista. :).