Gnash is a free Flash movie player for Ubuntu which works as a standalone player or as a plugin for Firefox and Konqueror.
This is the default (I believe) player installed on Ubuntu 7.10 (the Adobe player isn’t there by default).
I recently tried to load a Flex app in this player and failed, the reason is that Gnash is in alpha state and supports Flash version 7 (and some AS classes for 8.5) only.
Of course you need Flash player 9 to run Flex 2+, so you’ll have to use Adobe’s Flash player for now which thankfully works fine :)
To install the Flash player from the terminal:
sudo apt-get install flashplugin-nonfree
Or to try Gnash:
sudo apt-get install mozilla-plugin-gnash
This will be another interesting project to keep an eye on.