Oh, the flash tags? I think that is a SMF thing.
I believe most other forum systems can do it too (I could be wrong on both of these).
Though, I bet most admins disable it.
Could you imagine a feature like this enabled with the maturity level most have on FacePunch for instance?
Dumb songs/flash games/ads would be everywhere.
Once in a while is cute and fun, maybe even informative if a swf file was found that shows something interesting (building something, etc)
But all it would take is one person trying to add something, even this cute smiley, as his signature on every post, and an already questionable forum would become intolerable.