One of the missing features in jabber most commercial instant messaging systems have are small games one can easily play over them. Till now, because just yesterday, gajim got support for Tic Tac Toe over jabber.
Which reminds me tic tac toe was one of the first »programming projects« I did in the past, some day I'll have to dig out the sources and publish them. My next wish would be something like tetrinet over jabber.
There are previously existing tic-tac-toes over Jabber, but yeah, probably not integrated into the client directly (which I find to be a bad idea, but I digress).
For instance http://blogs.gnome.org/xclaesse/2008/04/21/announce-empathy-0231/