OS X for my editors, browsers, and VirtualBox. Vagrant for managing the VMs that my work actually runs in, with varying versions of Linux. Gives me lovely window management and polished apps and utilities (plus great integration with my phone and tablet), but all the power and comfort of a linux commandline for work!
Windows. With an Ubuntu server running next door as a dev server/filestore. So all code is executed under Linux, Windows just provides the tools.
Vagrant going on the next Windows box I get.
OSX. I just find it more natural to deal with switching windows and setting up tools. But I'd switch back to Windows if it was easier to get things up and running for web dev.
Mac osx because I like imac hardware and linux-y feel of it.
Linux.
Emacs and git with the magit package.
Node, meteor for fun projects.
apache, postgresql, and php for less fun projects.
Ubuntu for years. The packages system is so convenient and it works out of the box. However I compile Ruby my self.
Windows 8 + Vagrant for projects that run better on Linux (node/ruby/etc).