The next Google
Business 2.0 has an article about the Next Net 25.
Not to many interesting companies though. There is a Social Media category with Tagworld, Digg and YouTube.
I have a serious problems with the content of the Social Media sites. They are populist and in many cases they favour stupefying and sensational content. Sure that can be fun, but it can not beat a good editor.
There is also a mashup category. Mashup sites will always be a marginal fenomenon. They can not grow beyond the sites that they are mashing. Also, since most mashups are done on data that is publicly available there will always be fierce competition.