If you've got some time on your hands and fancy some unique and decent-sounding speakers, then why not make them yourself out of concrete? That's exactly what Markus Egger did, making his own frames for poured-concrete and using wax blocks to hollow them out. Apparently the high-density of the material means that bass response is particularly fine.
If it were me, as good as the painted results look, I think I would've left it raw concrete - very cool in a minimalist decor kind of way. Full project log at Markus' site.