Just finished this today for a client.  It's a diatom (unicellular algae) called Didymosphenia geminata, aka Rock Snot. In the background are smaller versions of the cell on the stalks that they use to attach to rocks.  Sure hope I got that description right.  I like that it turned out looking a little bit like an alien monster.

This design should be available on a t-shirt pretty soon.  If you're in to diatoms, you might be able to buy a t-shirt from Rhithron Associates

And if you want to know why they call it Rock Snot, this should be pretty well self explanatory...