Even though I'm a great fan of Windows, especially being fully satisfied with the upcoming version (hasn't RTMed) of Microsoft’s latest OS – Windows 7. I occasionally use linux, specially to try out certain code with c++ and a bit of traditional assembly code in using system calls.
Ubuntu my favorite distro of Linux, is available for free to download as any other linux from its official website here. What surprises me more is that you can still order the Ubuntu Desktop Edition or Server edition without paying a penny, even after a month of its official 9.04 release last April. Click here to order yourself a copy.