Links: Animated Knots
Animated Knots by Grog is a one-stop place for learning how to tie useful and safe knots. I know, I know, you can Google about 70,000 sites for tying knots, but let's face it: a series of black and white pictures with numbers affixed to them does not make it easy to learn to tie the darn things.
Especially helpful are the explanations about what the knots are supposed to do. Some knots can be tied with no load, and some knots must be unloaded to untie them at all (the bowline is a good example). If you've ever had to tie down a bike on a car roof and struggled to make it secure, you might pick up some pointers from this site.