I have written about do it yourself extension tubes before, but I came across this instructable where user BorisKafka basically built themselves a macro lens using a redbull can as a extension tube.  It is an extremely easy instructable.  They took a camera body cap and cut a hole in it.  They then glued a hollowed redbull can in front with a cheap almost disposable camera lens on a paper slide in front of that and presto! a cheap macro lens for little effort or cost.

The results are not to shabby either:

I would like to encourage you to check out the instructable at: http://www.instructables.com/id/Ghetto_Macro_Lens/