Ross Mayfield (see link below) wrote a nice review of social software. If you find something interesting in the following list, follow this link for more information.
Ning - "aiming to be the mother of all social software"; Flock - "aiming to be the browser that we always wanted"; Wink - "incorporates user tagging and ranking to provide recommended results and block spam"; Memeorandum - "dashboard view of blogs and MSM with the ability to drill down into conversations"; Sphere - "good incremental improvement over existing blog search engines"; Rollyo - "create personal search engines using only the sources you trust".