Parallels is one of the most popular pieces of virtualization software. An update has been released to make sure that it runs properly on Mac OS X Lion. The update for Parallels 6 is free. However, if you are running Parallels 4 or 5 then you can upgrade to 6 for $39.99 for a limited time.
To upgrade Parallels choose Check for Updates from the Paralells menu. The latest version is 12094 and weighs in at around 208 MB. Be sure to update your virtual machines after installing the new build.