Add whatever I've missed. I'll revise this top post every now and then to reflect whatever people post.

Just a beginning...

General Knowledge and Useful Technical Information
Sheldon Brown's Big Happy Funtime Fixed Gear Knowledge Repository. A First Stop.
Fixed Gear Wiki (help them and add what you know)
fixedwheel.org.uk - Articles, How to's etc.
63xc Fixed Gear 101 - Riding Tips
Fixed Gear Gallery - Links, Articles, Forum, and photos of bikes from around the world.
Old Skool Track - Articles, Pictures, links, emphasis on bikes from 70's and older.
Fixed Innovations - Tips and howtos for conversions
Bicycle Safe - Tips for not getting flattened by a car
Wikipedia's entry on Fixed Gears
Fixie FAQs Wiki
Puma's Fixedgear 101 Booklet - If you buy this off eBay I have some wonderful waterfront property...
Jim Langley's site - Many interesting tips and articles tho not FG specific. Lots of history
The FiXed Gear
MacG's Gearing and skidpatch worksheet. Nice

Where to buy crap online (SUPPORT YOUR LOCAL BIKE SHOP)
American Cyclery (San Francisco based)
Business Cycles (Miami)
Harris Cyclery (Mass)
Trackstar NYC (NYC)
Nashbar
World Class Cycles

Component Makers.
Campagnolo
Miche
Shimano
Mavic
Brooks Saddles
Phil Wood
Chris King

Frame Builders/Bike Manufacturers
IRO - Frames, Complete Bikes, Components
Don Walker - Bike Forums regular + Frame builder
Kogswell - Another BF regular. Doesn't build their own frames but they sell some nice ones and some other products.
Jonnhy Cycles
Vanilla Bicycles
Yamaguchi - official framebuilder of the United States Olympic Cycling Team
Spicer Cycles
Bernie Mikkelsen
Independant Fabrications
Kalavinka

Bags + Gear + Clothing
Bailey Works
Reload Bags - also have top tube coverings and other miscelania
Chrome Bags
Bagaboo Bags
Under the Weather - Bags and Clothing
Rapha - Superfancy expensive clothing + gear
Pac Designs
timbuk2

Velodromes
Kissena (NY)
Hellyer (San Jose, CA)
Alpenrose (PDX)

Tips and Tricks
How to do a cool messenger style handlebar grip
How to do cloth tape + Shellac
How to do a 2 color weave pattern on your bars
Butchering a Brooks