Microsoft has been pretty generous in letting folks play around with Windows 7 for free. But it has put some time limits on that generosity.
Those who want to try out the release candidate of Windows 7 only have until August 15 to download the code. After that date, you can still install a copy you have downloaded (and even get additional product keys), but most folks won't be able to get the code from Microsoft's Web site.
For those who have a spare machine to try out Windows 7, it's worth a try. I have been using it for months now and stable enough to use as my everyday machine. It's a particularly nice option for Mac users who have wanted to try Windows on their Mac but haven't wanted to steal a copy from work or fork over the money to be legit.
A separate deadline is looming for those still running the beta version, as opposed to the release candidate. Starting July 1, that software will start shutting down every other hour. (Here's the place to insert your best anti-Windows joke. I'll get you started: "Why, that makes it just like Windows ME.")
In any case, to avoid that nastiness, one should move from the beta version to the release candidate version in the next seven days or so. In addition to the shutdowns that start July 1, the beta software will expire completely on August 1.
Similarly, the release candidate also won't last forever. Microsoft says it is set to expire on June 1, 2010, with the bi-hourly shutdowns starting March 1. And, Microsoft also says that those moving from the release candidate to the final version of Windows 7 will have to do a clean installation, so be prepared for that, as well.