This is one part of the UCS taxonomy chart with examples of UCS's that aggregate people's opinions or ratings.

Zagat Guides

Zagat Guides were one of the first user contribution systems, taking restaurant reviews out of the hands of paid critics and giving ordinary people a "vote" on the quality and value of their local dining venues.

• See the write-up in this wiki for more details

Yatt'it

Yatt’it allows Hyatt Concierges and Gold Passport members to provide and rate “insider tips” for fellow frequent travelers (assumed to be likeminded) and get up to date flight stats

• Yatt'it home page
• Write-up in tourism and hospitality blog 

Netflix

Netflix encourages users to rate movies they've watched. Their algorithm then identifies users with alike tastes and recommends movies based on this.

• Netflix
• Netflix (Wikipedia)

Rate My Professors

A site where college students can rate their professors in multiple categories.  Also, give professors the ability to "strike back".

• Facebook page
• Rate My Professors writeup as one of Time's 50 Best Websites 2008
• Could RateMyProfessors.com Be Right? looks at the results of a study showing the site's ratings "have a significant correlation with the formal student evaluations"