One of the simplest and handiest utility items for brewing.
I got a couple of 2x3s plus four fairly heavy duty castors, cut seven 14" long pieces (pick your length based on your gear), smoothed and rounded everything off and screwed the pieces together, five across, about 1/4" between each piece, laid across the other two. Two deck screws per joint. Then I attached the castors.
It cost me about ten bucks (most of which was for the wheels - you could go cheaper but I wanted tough ones) and took me maybe 20 minutes total. I have a nice heavy duty dolly that is just the right size for a five or six gallon fermenter. Plus it has plenty of other uses around the house. No more lugging a full glass carboy around and worrying about dropping it.