I am in the process of writing an article on comment platforms, what I am calling Web 2.0 services like Digg.com, Flickr, YouTube, etc. These are all the places you can post news, photos, videos etc. and people rank and comment on them.
I strongly advise PR people to monitor these sites, to see what is being said about their companies and products. It gets more tricky, however, when PR people want to actively intervene, posting a comment, for example. Recent kerfluffles on Wikipedia, Digg.com and other places where PR people are strongly advised to "look don't touch" make it even more important to approach these places carefully.
If you have a success or failure story about your PR activities on these various comment platforms, please drop me a line at ealbrycht at gmail dot com or leave a comment below in the next few days. Thanks!