Crossover, with a capital 'C', is a rather nifty plugin for Linux web browsers such as Mozilla, Netscape, and Opera. Made by Codeweavers, it uses Wine to enable these Linux browsers to make use of Windows plugins--most notably Quicktime, thus addressing one of Linux's greatest inadequacies since the introduction of the Sorensen encoder.
Crossover is a mixture of both open source and closed source software, and cost $20 with a nagware demo. It works amazingly well on a 450 mHZ overclocked Celeron with 192 megs of RAM, running movies just like you were in Windows. It is rather odd to see the Windows installation wizards running in one's X session, however.