A couple of more not mentioned

Street/urban - Ironically probably the most old school riding that has moved into mtb. Essentially a bunch of mtbers trying to do what bmxers do but on mtb's. Mtbers will eventually find their own niche in street but for now come up short compared to bmxers. Generally the mtbers also merge the hucking aspect of freeriding into the scene. Grinds, pedal kicks, lots of tricks all make up this aspect and it is very cool to watch.

Trials - the most technical form of riding EVER. Yes thats my opinion but these guys rock. They have more control over their bike than most people have over their bodies.