My computer runs Mac OS X (a branch of UNIX) and Windows 7 in a VMWare window. It's a great working environment I highly recommend – you get the three best operating systems on the planet in the same process space.
For translation work, a translation memory system is almost indispensable today. So far, I have been using Trados Studio 2011, which I use even for small projects.
Apart from software that I use only sometimes, like Idiom or Alchemy Catalyst, I also have developer tools at my disposal, such as Navicat for accessing databases, an XML editor, or Perl. I found these programs handy for fixing or patching tagged translation files.