Harbor Freight.
In 2007 we bought a lift table from Harbor Freight (~$100) and build our cart around it. It is wonderful to have in the shop back home and at competition. We can raise the robot to a comfortable work height no matter what we are working on. Northern Tool also sells similar lift table carts reasonably priced.
Edit Examples:
http://www.harborfreight.com/cpi/cta...emnumber=94822
http://www.northerntool.com/webapp/w...5546_200365546
Neither of these are the exact one we bought but are similar. They aren't large enough to be a useful cart as they are but expand their width, add some better casters, pimp them out with some polished AL diamond plate and they make a great cart.