Last year we saw an amazing bicycle that had been hacked so that it could change height on-the-fly. Like this:
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I could not deduce how it worked, but Rachel over at Rako Bikes figured it out--and even made an Instructable showing you how to build your own. You'll need a way to cut metal, access to a welder and a budget for some basic bike parts and tools--and, of course, a full-suspension mountain bike you're willing to hack up--but she makes it look pretty do-able.