Microsoft has been fairly generous in letting people 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 15 August to download the code. After that date, you can still install a copy you have downloaded (and even get additional product keys), but most people won't be able to get the code from Microsoft's Web site.
A separate deadline is looming for those still running the beta version, as opposed to the release candidate. From 1 July, that software will start shutting down every other hour.
To avoid that nasty business, you 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 on 1 July, the beta software will expire completely on 1 August.
Similarly, the release candidate also won't last forever. Microsoft says it is set to expire on 1 June, 2010, with shutdowns occurring every other hour from 1 March. Microsoft also says that those moving from the release candidate to the final version of Windows 7 will have to perform a clean installation, so be prepared for that, as well.