Gnash is the free implementation of Flash. Gnash is one of the projects that is deemed vital as it prevents people to have a completely Free system. It is vital because Flash is used for many applications that are really popular like Youtube it is also used a lot for educational software.
The inability of using Flash in MediaWiki is considered by most to be problematic. Having met Rob Savoye, I learned the first things of Flash and Gnash. Gnash is used widely, it is used for OLPC and it is used in many embedded systems. I expect that what Gnash serves, a Flash client can display.
the cabbage and the goat from the wolf :)