For funny websites, I have seen www.fark.com giving insane amount of traffic. Similarly www.dzone.com gave one of my client tons of traffic just because it was related to programming.

I have put this as a separate entry from Social Boookmarking to stress on the point.

For example, check out http://www.searchles.com/ to know there are other sites than Del.icio.us/Reddit too.

You can check this list to get a nice list of niche social bookmarking websites.