One of the best sources on the net for cycling esoterica. I practically lived on this site when I was putting together and fixing up older bikes.

Especially good information on practical fixed gears and three speed hubs.

Before he passed, I sent an e-mail asking a few questions and Sheldon responded personally, with fairly detailed responses. It was pretty rad.

There's a lot of really useful information on there.

Some of my favorite parts.

The three speed hub with a fixed gear: http://sheldonbrown.com/asc.html

The section on frame fit (semi controversial topic depending on the crowd): http://sheldonbrown.com/frame-sizing.html