Google were experimenting with voting on results:
http://techcrunch.com/2007/11/28/straight-out-of-left-field-...
Also there is this form for reporting spam sites: https://www.google.com/webmasters/tools/spamreport
Integrating the above into standard search results would be difficult unless it was restricted to users with a good "karma". That might be possible in our increasingly socially networked world