Confessional blogs

This group is reasonably easy to discern.
Diary-like entries detailing the minutiae of everyday existence.
Web dairies have been around for quite a while now and pre-date blogs (some would see them as the natural precursor to blogs).

These provide an outlet for the frustrations, joys, pain or the (creative) writing talents/needs of the producer without aiming to impact on the lives of others.
These blogs may well turn out to be important incubators for a new generation of writers and poets, just as the café/beatnik scene or fanzines were to previous generations.