The veritable ocean of available video formats can often instigate a confusing and frustrating adventure in hunting for plugins and explaining to grandma why that DivX email forward she received plays the audio, but not the video. Fortunately, instead of bookmarking the plethora of codec download pages and backing up loads of installation disk images, a new plugin called Perian brands itself as a "swiss-army knife for QuickTime," as it aims to help solve all our video format woes. Perian supports an impressive list of codecs, including DivX, XviD, 3viX, FLV (y'know, those Flash files you can save but can't play because Adobe's Flash player doesn't seem to install an actual GUI front end?), AVI, and much more. Of course, you still need Flip4Mac if you have to play WMVs, but it looks like Perian can have you covered for most of the other formats you (and grandma) might encounter in your journey across the internets. As an upcoming bonus for Democracy Player users, Perian will be included in the next version.
Perian is offered for free from Perian.org, though I can't track down any system requirements or whether it works on both Mac and PC.