I use Linux most of the time and windows the rest. I'm planning on switching over to Leopard sometime in the near future though. It's the unfamiliarity that worries me about switching OS's though.
90% Ubuntu 8.10 10% Windows XP Pro Sp3
It is pretty sweet. The debugger alone makes it worth it. Bashing IDEs seems to be in vogue for whatever reason. If you haven't tried NetBeans it is worth a look.
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
Debian GNU/Linux with emacs, git, capistrano, mysql, memcached, etc. In production, same OS plus Apache with Phusion Passenger.