We all know that YouTube and other popular video-sites use Flash as their multimedia file format. Whilst Adobe provides versions of their for Linux, the player is in no way open source. We Linux-users prefer using open source alternatives over proprietary, closed software, so the community has embarked on an open source Flash Player and named it Gnash.
Gnash provides both a stand-alone player as well as a web-browser plug-in. At its current version, it can now play YouTube videos and other Flash-based files that are based on version 7 and below. Support for Flash 8 and 9 are now being worked on.
Personally, I have been waiting for Gnash to release version 1.0 and hopefully get it working on Windows, Linux and Mac OS X. This will release me from dependency on Adobe products. :)