Silent Film.

Paul Auster's novel Book of Illusions made me want to watch more silent films.

I had both of the Chaplin box sets released by Warner and also a box of his shorts, and they were superb.

Tom & Jerry cartoons can be treated as silent shorts.

I am taking the Eclipse box of silent Ozu films with me to New York and hope to watch them in the semi-near future.

Have you seen The Passion of Joan of Arc?

Most of my favorite silent films are the later ones. Early ones have the power of magic (Mieles) or actuality (Lumiere). But the ones from the late 20s have the most fully worked out visual vocabulary. The most sophisticated film factories were in the US and Germany, which is why Hollywood imported so much talent from Germany and the rest of the world. Check THE WIND by Sjostrom, or SUNRISE by Murnau, or SPIES by Lang. Weimar era Lang was a pulp genius. DR. MABUSE is crazy, and METROPOLIS is crazier still. DER MUDE TOD (in english, DESTINY) has astounding visual effects. Murnau's FAUST is magnificent as well.

The Chaplin box sets are scattered across the country now, one of the casualties of my giving up most of my worldly possessions over the last month

Mack knows what's up, great recommendations. Especially the two Mabuse films (well, one film split into two) are amazing.

In addition to what's been posted, here are some that I'd recommend by various directors

Carl Theodor Dreyer. I think that visually Dreyer was operating on a whole different level from anyone. All three (and his later work too) are amazing films. Definitely one of the all-time greats.
-Michael
-Passion of Joan of Arc
-Vampyr (this was originally a silent, but converted into sound during post-production)

Yazujiro Ozu. I've only seen these two of his silents, but I love them both. And their remakes.
-I was born, but... (which he later remade as Good Morning)
-Story of the floating weeds (which he later remade as Floating Weeds)

Sergei Eisenstein. If one is interested in silent cinema, here's a good place to start. All of Eisenstein's silents are amazing and I would also recommend his writings on the subject.
-Strike
-Battleship Potemkin
-October

Aki Kaurismäki. Had to mention this, but this is also a very worthy film, not a novelty by any means.
-Juha

I'm sure that I'm not the only one who feels from time to time that the invention of sound wasn't all that good of an idea in regards to the development of cinema as an artform. Something gets lost the moment sound is introduced.

Oh, and as a general recommendation, I highly suggest checking out concerts where live bands are playing the music for a silent film. I've seen Eisensteins Battleship Potemkin and Strike and Chaplin's Modern Age like this and it really does elevate the film in question. Assuming of course that the performers are up to the task.

Not sure if this has been mentioned and I'm no expert on film, but I loved the Cabinet of Dr. Caligari.