There seems to be many trading card game projects floating around the Internet. I thought I would have heard about most of the major projects, and maybe I have, but here is another project called OCTGN which lets you play card games against other players over the Internet.
OCTGN supports Magic and major a few other cards games, although the documentation is nonexistent. I presume it does not enforce the rules but I could be wrong, again there is no documentation. It does support Two-Headed Giant would should be some fun, below is a screenshot. You can find more information by searching Google for OCTGN.
Main OCTGN Site