Microsoft has officially released Windows 7 Release Candidate to the public, which can be obtained here.
Unlike its Vista test releases, they claim to have no limit to the number of copies that can be downloaded and will continue offering downloads until the end of July.
The release candidate will fully work until March 1st, 2010. From that date, it will start shutting down the PC every 2 hours and finally expire on June 1st, 2010. In the meantime, this gives about 10 months (+3 months of 2-hour sessions) of testing time.
Despite this being the first day of this public release, the download speed was impressive at ~1.2MB/sec throughout the download in Firefox, which is about as fast as the connection at my work place handles and took about 45 minutes to complete. The ISO file is 3.04GB.