“Ubuntu Essentials”
This is old news by now, but still a handy reference. Mark Pilgrim has listed his most-valued applications for Ubuntu Linux.
Pilgrim was a Mac developer and blogger until he decided to switch fulltime to Ubuntu. (Not sure if he stuck with it or not, though.)
I plan to remain dual-platform, focusing on both Mac OS X and Linux (Ubuntu, in particular). I use Windows when required, but never by choice. Aside from the occasional (and ever more rare) application only written for it, Windows has no advantages and some major disadvantages when compared to Linux and Mac. (Viruses, high price, awkward UI, etc.)
In any case, the list of applications Pilgrim prefers is informative and very interesting (and many of them are cross-platform).