I've put up some google custom search engines by collecting the 20-40 most active delicious taggers for a topic, e.g. clojure, and counting their tagged domains.

Works ok, a little better precision, lose some recall vs. generic google searches, but need to renew the list of domains pretty frequently

For people that don't know what a vertical search engine is: http://en.wikipedia.org/wiki/Vertical_search

Usually Google using the "site:" command for stackoverflow etc, since I'm satisfied with the way Google orders the results.

If once every month or so counts.

Google to search wikipedia, stackoverflow, imdb

Octopart

stackoverflow, twitter search, searchyc, facebook.

markmail, krugle, imdb

crunchbase, glassdoor, imdb, (clerkdogs), twitter, backtype.