My feet. I commute to the basement.
When I had an office to commute to I started with a huffy mountain bike. Moved to a hybrid and from there to a touring bike. Once that was destroyed I replaced it with a fixed gear that was also destroyed and then replaced with another fixed gear. That bike is being replaced with a cx bike as I have grown to despise the whole one gear can't stop pedaling BS. Though I suppose I could throw a free wheel on it instead.