BikesDirect Single Speeds
The link above is a nice, inexpensive start to the world of SS/FG riding. I personally own a 2005 Specialized Langster. Other types would be the Bianchi Pista (no brakes), the Bianchi San Jose (will fit fenders and larger tires), the Specialized TriCross (SS model), the Redline 9-2-5 (fenders included), and the Giant Bowrey (sloping top tube). The Surly Steamroller is also an option.
I still would love to have a San Jose because of the pure utility of that bike. You can do it up as a brakeless road fixed gear or set it up as a fendered, freewheeling, fat-tired commuter with panniers.