Keotag is a convenient way to search for stuff (not just tags) across all popular blog search engines and social websites including Twitter, Digg, delicious, Flickr, YouTube, etc.
You type a search term (or tag) and click the corresponding search engine icon where you want to perform that search. Keotag is AJAX based and so you can switch search engines without having to reload the page each time.
No options to sort or refine search results but still quite useful especially when ego-surfing. Tag Viewer is another competing site but I would prefer Keotag.
Related: Search Engines That Are Not Google