WikiSeek is a new Search Engine ("obviously" marked with the ubiquitous BETA word) that queries WikiPedia - and all the external links that WikiPedia references.
In this way the results it gets are better (or at least more complete) than the results you get from WikiPedia itself. And it features another web 2.0 thing: the tag cloud, in this case built with WikiPedia Categories resulting from your query. This category cloud fills up with suggested categories as you type your query - I like this feature.
Finally there is a plugin you can run in order to add WikiSeek to your search providers (in IE7 or Firefox, at least).
Not bad at all. I'll use it a lot, I think.