There is no exercise out there which will improve your climbing other than actually getting on your bike and hammering those hills and climbs. The best way to become a strong climber is to do a lot of hill climbs. One thing I can recommend is riding a singlespeed rigid forked MTB and use that to become a stronger climber. I've been riding singlespeed MTBs since 2008 and it has really made me a better climber and improved my overall bike conditioning.