I followed the links on the NLA's cloud on Del.ici.us for family history and found it quite useful at a very basic level. However, there were an awful lot of links to follow to get to the basic 'building blocks' of genealogy such as indexes to births, deaths and marriages.
Tags are interesting. However, to adequately categorize a multitude of websites they are quite crude. With no common thesaurus of terms to use in tagging it is easy to see how you can miss many items that might be in your area of interest. The alternative is that it is easy to be overwhelmed by too many 'hits' because of the generalized terms used for tags.
LibraryThing is great fun and obviously has great application for book clubs or any group with shared reading interests. However, it is not particularly intuitive to use and I totally failed to be able to add My LibraryThing to my blog (it didn't help that my 'add a gadget' option disappeared from my blog settings and I couldn't work out how to get it back.)
Shelfari was a much easier option both to navigate around and to add to my blog.
