Try looking for places where there is quality content and people who would enjoy that content if only they could find it. Where this occurs, there should be lots of potential to generate value, by matching people with content they might like.

For example, last.fm does this for music, del.icio.us/digg/reddit/news.yc for content, and I'm sure there are lots of other examples of services like this.

Here are two areas where such services, to my knowledge, don't exist: web comics and blogs.