You can do a lot of things with cinder blocks, but creating a set of speakers is one of the less obvious options. Cinder Speakers are a development from Daniel Ballou and Dashdot that turns simple cinder blocks into high fidelity speakers. It turns out that concrete is a low resonance material that minimizes vibrations and reduces coloration of the sound delivered by the drivers. They ship pre-assembled and pre-wired, including a 5" driver, tweeter, crossover, and input jack. Of course, they are heavy, but once set up are able to create an impressive sound given the rudimentary components.