Let's say you found a great website, but you want to see some other, related sites. Check out inSuggest. This site lets you add URLs, then suggests similar sites based on content. You can also get image suggestions, based on the images on chosen websites.
For even more personalized customization, you can get suggestions based on your bookmarks. Click the tab, then enter your delicious username and you'll get suggestions based on your bookmarks. It's pretty powerful stuff.
Besides being a fun tool, it's also a good way to research your competition, or find some new resources for your own use.