Lately, its been Faraday's law, the Fermi-Dirac distribution, the Bose-Einstein distribution, the density of states for a 3-D infinite square well, and the Boltzmann distribution.
Gotta love quantum statistics. Well, not really, but they make it sound fun. At least, they try to make it sound fun, but its really not. Especially at 3 in the morning when you have homework due the next day, and you can't figure out what equation your supposed to be using, and nobody else in the class knows either.
Andrew