I take Brazilian Jiu Jitsu. I've been taking it for 4 months but plan to keep at it forever. For those who don't know what it is, it's a ground based grappling art that focuses joint manipulation and choke holds. It's extremely effective. And for all nay sayers one of the Gracies who created it from Japaneses Jiu Jitsu, won 3 of the first 4 Ultimate Fighting Championships and it is still a huge part of any mixed martial arts fighter.
In my personal opinion it is the most powerful grappling martial art, and more powerful than almost any other striking martial art in one on one combat.
But I'm still looking for a striking art to combine with my ground fighting.