Around the time of the release of Ubuntu 7.10, I tried out the Gnash Flash player included in that release. Because Adobe’s Flash player can not be redistributed in the default Ubuntu installation, a choice of players are offered when you visit a page in Firefox with Flash content. There are three options in Ubuntu 8.04: Adobe Flash, Swfdec (new to this version), and Gnash.
Gnash is an open source alternative to Abobe’s proprietary Flash player. It has support for the older SWF v7 Flash format and SWF 8 and 9 are in development. Gnash is part of the GNU project. Swfdec is a similar project licensed under the LGPL.
I started both of my tests by browsing to a YouTube video page. Firefox would prompt me here to install a plugin to view the video.