In this video, Dr. Jaak Panksepp describes how he discovered that rats apparently laugh, producing an ultrasonic sound which more resembles chirping than laughter as we typically think of it:

Further research by Dr. Panksepp has shown that young rats tend to be more ticklish that older rats and that "laughter" ceases when the animals smell the odor of a cat or when they are hungry. As you might expect, his findings have been debated by other scientists.

To read a recent interview with Dr. Pankseep about rat "laughter" and his theories, click here.

Sources:

Scientific American

http://falk.mccormick.northwestern.edu/doc/07LaughingRatsPlayfulTicklin…