Sproose is a relatively young search engine that now allows its users to control search results via voting. Each search result gets its own vote count and the option to click "I like it," which brings the result up to the top of the heap. The obvious comparison here is Digg.
Sproose searches through videos, using Blinkx, although these results cannot be re-ordered or voted on. Users can also opt-in to have their votes recorded on their profiles for others to see and vote on. All results get their own comment area, where users can talk--although I doubt many will want to write about a search result.
One of the more interesting inclusions in Sproose is its tag cloud, where users can browse for sites via tags. This is kind of an interesting form of Web discovery service, although it's lacking a clear way to add tags to sites yourself.
The obvious concern with sites like these is gaming results. Sproose deters some of this by removing votes from unregistered users after a limited time and requiring registration, although savvy spammers can usually find a way to circumvent whatever verification processes exist. Sproose's results are only as good as its users, and if security fails, so does the site.
See also: aftervote