I've managed to build my own rotating composter for about $50.
I've always wanted one, but they cost about $300 commercially.
The idea is that a rotating barrel is easier to aerate by simply rotating the drum.
Aerobic microbes are much more efficient at composting,
so you get finished compost much faster this way.

I found the plans for one from the
Rodale Institute. Rodale does great work in the area of sustainable agriculture.
The plans can be found
here in pdf format.
If you're following the plans, I would recommend replacing the 28 wood screws, #10 x 1.5", with #10 x 2", cuz the shorter ones won't go through two pieces of 2 x 4.

A photo of the cross brace support. A jigsaw does this job wonderfully.

A shot of the top corner. Be sure to put the top horizontal piece on the outside of the legs.

The 0.5 inch notch in the top horizontal piece.

Cut a door in the side and attach a latch and hinges.

Close up of the latch that I used.

I made the bearings out of square pieces of wood. The larger bearing will be bolted to the barrel, and the smaller bearing is glued to it. The small side goes against the frame, i think to limit friction and so that the bolts don't catch against the frame.

Glue the little bearing to the bigger bearing.
You can see the piece of rebar that will go through the barrel and hold it in place on the frame.

The whole is drilled in the "exact" center of the barrel, so that the rebar can fit through and rest on the frame. I got the barrel from a friend for free.
The project should be finished in a day or two, once the glue dries. I'll post photos of the finished product.