The model organism
Caenorhabditis elegans is a small worm (1-mm long) that can be isolated from rotting vegetable matter, feeding on the bacteria present on it. C. elegans is a powerful system for cell and developmental biology, neurobiology and genetics.
Numerous resources have been developed for researchers working with this model system (wormbook, wormbase, wormatlas, wormweb).
You can learn the basics of C. elegans in this Introduction to C. elegans