The Ecosphere is a sealed globe containing algae, shrimp, and gravel. You control the temperature and the amount of light it receives, otherwise, the system can sustains itself for several years! An abrasive magnet can be used to clean the inner surface, if neccessary. Useful as a learning tool or as a very low-maintenance pet. http://www.eco-sphere.co.uk/ -via Dump Trumpet
I do however, have an Ecosphere myself which I purchased in December of 2001. Of the three shrimp inside, I still have one that's still alive and kicking. When that one dies, the Ecosphere people will "re-charge" the sphere with new algae and shrimp for a nominal fee.