If you have any experience with sites that use tags (eg. Flickr, Last.fm, del.icio.us, Technorati), you’ll be able to pick up Tagos quickly. Users can create items (which are primarily text, but can be fully formatted with headers, emphasis, images, links, etc.), tag these items, and search for items based on their tags. Items can also be editable, wiki-style.
It’s currently in early stages of development, but that’s no excuse not to check it out and give me some feedback. When it gets to a high-enough level of quality (the code, not the content), I plan on releasing it as open-source.