The things that I enjoy doing the most with Flash, is to setup simulation experiments. Toying around with fresh gameplay ideas in the process. Recently I came across download-able prototypes to the game Spore. It's interesting to see how simplistic some of these prototypes look, yet I find them very inspiring. they also mention as inspiration to one of their prototypes.
I'm sure there are many more experimental websites out there.
If you have any favorite websites that showcase experimental game ideas, please share them below. Also discuss which inspiring prototypes have sprung up side projects or even full fledged games.