Leap is a new Finder replacement from Ironic Software that eschews traditional directory structures in favor of search and tagging. The software is pretty smart, and groups together common file types so you can find what you're looking for in a hurry. It also allows you to add your own tags to files, and creates a 'tag cloud' that outlines what kind of files you're looking at, what folders those files are in, and what user created tags they possess. The larger the label, the more files are tagged with that (as you can see in the screenshot above I have lots of image files on my Desktop).
The iPhoto-like UI is easy to use, and really shines when the Loupe is used. The Loupe is a tool that lets you peek at the contents of supported file types without launching any additional apps. Leap is in Beta, and the Loupe's performance makes that clear. It was a little slow, and sometimes lagged when I pointed my mouse from file to file.
In addition to all its Finder abilities, you can use Leap as a replacement for Apple's built in Spotlight interface, and yes the tag cloud is right there with your Spotlight results.
Take a look at this demo movie to get a better idea of what Leap can do for you.
Leap is currently in beta, and pricing has not been announced. You will get a free license, though, if you buy Yep, a PDF organizer that Ironic sells for $34, now.