In reply to Steve Ramsden: If the angle of the slope is right and the conditions are OK, you can "carve" a mountain board very similar to a snowboard. However, I found that if the tyres aren't particularly grippy on the surface it can "wash out" in the middle of the turn and you can crash to the ground VERY painfully. If you wer going to do it purely to improve your snowboarding I wouldn't bother, but OK if you are looking for a bit of fun. I would go to a centre and try it out first, before buying.
Any board sports have a certain overlap skate/surf/snow/mountain as the balance is similar. Indo boards are quite good for improving balance for boards.
http://www.magicmotionuk.com/ see "surf wheely scooter" looks a lot more fun though.