Denis Sinegubko tells me of his new tag searching facility in his software FirstStop WebSearch. Here’s an excerpt from his FirstStop Blog: Social Bookmarks in FirstStop WebSearch which explains it in more detail:
The pre-installed (in version 4.2) category “Social Bookmarks” contains the following search sources: LookSmart’s Furl.net, CiteULike.org, and Zniff.com, a search engine for the Spurl.net. Anticipating your question about del.icio.us, I can tell you that we didn’t include this very popular social bookmarks manager only because it doesn’t have a search facility.
Sounds like an interesting tool.