Quintura is the most recently updated and most useful of the tools. It creates a tag cloud based on your search term, and when you hover your mouse over a term, it dives into the term and gives you a more detailed tag cloud around that. A standard list of Web search results show up in the site's right-hand pane, and it refreshes automatically when you hover over links in the tag cloud. Only when you want to actually visit a Web page do you click on a link.
Quintura is easy to use, easy to understand, and does a good job of showing you the shape of a field of knowledge. There's even a kids' version. As with other search visualizers, it's very good at helping you to narrow down search results if your initial query is ambiguous. For example, if you search for "Dvorak," and you're looking for information on the composer rather than the alternate keyboard layout, you can quickly drill into your results; you'll simultaneously see a useful cloud of keywords and can filter out what you don't want.
Clusty also clumps results, but it does so in a more traditional format: an expandable, outline-like list of subtopics to the left of your results. It's less weird, and less fun, but just as useful as Quintura. It's probably better suited to people with very organized minds. Outliner fans will like it.
Clusty also has a Wii-friendly version, which is a neat trick.
Kartoo reflects a search concept that was in vogue a few years ago: Cartographic search. This site puts its results on a background that looks a bit like a topographical map of the ocean's floor. It's supposed to show you how "islands" in your results separate from your mainland of hits. Kartoo also has other clever features. When you hover over a site on its map, it draws lines between the site and other related sites. In the left-hand pane, it shows you either a changing topic list (Clusty-like) in the left-hand pane, or a visual site preview, depending on what your mouse is hovering over. Despite the richness of the data it returns, I find the interface confusing. There's too much happening on Kartoo, and it doesn't lend itself to quick comprehension of a field.
Kartoo has other interesting search visualization projects, including Ujiko, which to me looks like a search engine that the Star Trek set designers would build. It shows results in a bizarre oval menu with an open center section where the user can flag particular results as relevant. This input helps narrow down results. The center section also shows how terms in multiple results relate to each other, using a wheel of color codes attached to individual results. I think you need Spock's brain to interpret the data. That said, it's entertaining to use and does return useful and very specific results.
Grokker displays search results as bubbles inside bigger bubbles. The bigger the bubble, the more results (Web page hits or sub-bubbles) inside it. It is very easy to zoom in and out of subcategories to get a good understanding of a result set. The interface is fluid and Grokker is fun to use. A control panel, which is unique to this engine, lets you refine date ranges and data sources. The company's main business is providing search technologies to big companies, but Grokker is a good consumer search visualizer nonetheless.
I haven't integrated any of these tools into my daily routine, but at first blush Quintura looks to be the most useful, since it gives both easily-understood overviews of a field as well as specific Web page results, without overloading on a bunch of new search concepts and interface tricks.