RealPlayer

RealPlayer can easilly be instaled on the xo:

Enter the Terminal Activity and type the following:

wget http://software-dl.real.com/30797ed883585619ce21/unix/RealPlayer11GOLD.bin 
chmod a+x RealPlayer11GOLD.bin 
./RealPlayer11GOLD 

this will install it in /home/olpc/RealPlayer/realplay

to run it type

/home/olpc/RealPlayer/realplay

NOTE: realplayer DOES NOT work in browse, you will have to use the Firefox activity