I'm a big fan of Websitesthatsuck.com, especially their annual hall of shame. I go there when I feel depressed about my own web shortcomings - nothing like a dose of schadenfreude to brighten the spirit.
Here are just a few of the many useful lessons from the list of offenders this year:
1. Keep it clean and easy to read-busy is bad. Clarity and simplicity are huge not only in website design but also in the messages you try to get across.
2. Have one, clear focal point (and, I'd add, one clear call to action) or else you're making it very unpleasant for the visitor. Don't believe me? Visit the Salt Institute (warning: take two Tylenol first).
3. Good navigation is essential and must be from the perspective of the audience, rather than your own. Therefore, don't make your website sections into departments of your organization; make them what a visitor most wants to see.
I could go on and on, but they already did. There are more than 100 more tips on their site-check out 148 Mortal Sins That Will Send Your Site to Web Design Hell.