for playing most video formats i use mplayer since i started linux !
u can ge it frm
http://www.mplayerhq.hu/design7/dload.html
and ge the latest codecs package from the same page above
extract the codecs then compile mplayer ..
that will do the jb i think
for playing online videos ,my firefox that shipped with fc6 can view online videos .. //as am using it and it never letme down on porn
//
having any special site that u can't view it's videos ?