It seems like every fly fishing Web site has a page that describes tying basic fishing knots, but there are only a few that are not hard to look at. Here are a few of the better ones:
Global Fly Fisher Knots
Fly Fisherman’s QuickTime Knot Series (requires Quicktime)
3M’s Fly Fishing Knots
FlyfishingConnection.com
LetsFlyFish.com
Flymart Online
WashingtonFlyFishing.com