Microsoft is offering a free download of Windows 7 Release Candidate (RC) for the next little while.
According to the site, it will expire on June 1, 2010. However, starting on March 1, 2010, your PC will begin shutting down every two hours. The official retail copy should be available around October of this year, so you probably shouldn't be using it past then anyways.
Note, while a release candidate is considered a more stable version than a "beta", this version still has minor bugs and issues. Personally, I get the dreaded "blue screen of death" every time I open a DVD in Windows Media Player. However, this should not prevent you from downloading it. I've used Windows 7 for about 2 months now and I love it. Let me know in the comments what you think of it.