I carry this
Topeak Ratchet Rocket Lite Tool | Chain Reaction Cycles

It's small, super awesome, especially for installing bottle cages where clearance is tight otherwise. The only thing I have used it on the road for was readjusting my saddle height, but it is small enough to fit in my small seat bag (not one of the sagging ball sack kind) with a spare tube, tire levers and an extra CO2. I will also carry one when testing out a bike I have just finished working on so I can make all of the final adjustments. Other than that, I haven't used mine on the road.