Here's a fun little tool my brother seems to be involved in Little Big Cloud. Each url on the domain is a tag cloud - if you go there you're prompted to provide more tags and they automatically link to each other. I think the concept is great. Add typed links and you have a semantic web on one page. I think it is a little annoying it keeps asking me for keywords everytime I go to the same page (I'd rather see a seperate url to add, also from a search engine perspective) and some filtering for double keywords from the same user would make it less spamable, but other than that I think it is a nice little hack.