Intro: Upper Kitchen Cabinet Jack
I saw these at this site: http://www.woodworkingtalk.com/f17/cabinet-jack-4053/, and decided to try them out. They are hands-free, all the adjusting can be done with your foot.
Scaffolding jacks, angle brackets, hose clamps, 1 1/2" pipe, plywood and hack saw.. JB Weld and pipe cutter optional.
I didn't end up using the bolts, as I didn't want to drill through galvanized pipe to secure the angle brackets.
If the base cabinets are already installed, or if you prefer to put them in first (because you like to climb on them to do the uppers) then cut smaller sections of pipe to use the jacks on the counter tops.
Doing the uppers first also avoids having to do something to avoid damaging the countertop with the jack base.