Leucine (LEU or L) is a hydrophobic amino acid. Its shape in Foldit is similar to those of glutamate (glutamic acid) and glutamine. Use the CPK or EnzDes coloring options (in view options) to help tell these amino acids apart. Leucine can also be distinguished by its having one fewer "bend" before the terminal fork than glutamate or glutamine.
More information can be found at http://en.wikipedia.org/wiki/Leucine