Points/voting based systems
Slashdot implemented a system of user-based moderation of content and "karma" for users. Randomly selected moderators can vote on reviews and comments on the site. Users gain "karma" based on votes their comments get. Users with high "karma" have higher chance become moderators. "Moderator" is temporary position, they are randomly selected within users with high enough karma. Each moderator gets 5 moderation points, when all points are used - user loses moderator status. Same user can gain moderator status again later.
Every comment can have a score from -1 to +5. Initially each comment given a score depending on its author: +1 for registered users, 0 for anonymous users , +2 for users with high "karma", or −1 for users with low "karma".
Moderators can upvote(+1) or downvote (-1) comment and also attach a description correspondint to choosen vote - normal, offtopic, flamebait, troll, redundant, insightful, interesting, informative, funny, overrated, underrated. So comment can, for example, have rating "+1 insightful" or "-1 troll".
Score is displayed next to each comment. Users have an ability to set preference - which comments they want to see when browsing website (comments with score = or more then x).
There is also "meta-moderation" system of "moderating moderators".
From slashdot FAQ:
What is karma?
Your karma is a reference that primarily represents how your comments have been moderated. Karma is used to determine who moderates and who doesn't. You can improve your karma by posting intelligent, funny, informative or comments generally impressive to your fellow readers.
Karma is structured on the following scale "Terrible, Bad, Neutral, Positive, Good, and Excellent." If a comment you post is moderated up, your karma will rise. Conversely, bad karma usually indicates a user account used to spam or otherwise hurt the discussion.
Factors besides moderation also affect karma. Having a story submission accepted raises your karma. Also, meta-moderation can cause your karma to change. This encourages good moderators, and ideally removes moderator access from bad ones. Don't worry too much about it; it's just an integer in a database.
Users can vote on answers. Each answer will show number of votes and who exactly voted for it ("140 votes by X, Y, Z (more)") - more link will expand the list to show everyone who voted for an answer.
This voting influence the order of answers shown on a page:
Answers with more upvotes are ranked higher.
Answers with more downvotes are ranked lower.
A vote from a user who has written good answers in the past carries more weight (both upvotes and downvotes).
Answers written by users who have written good answers in the past will be ranked higher.
Votes from people detected to be gaming the system (vote collusion, spam, bullying, etc.) will be ignored or minimized.
Whether a user is an admin or reviewer does not affect the scores in this system.
There is also system of credits which let people use "Ask to answer" feature.
Ebay, Amazon etc.
These type of sites usually implements review system where person can rate a product and/or seller using 5-star scale and write text review for the product. Other people can vote on "helpfulness" of the review. Results of this voting shown on user profiles - "Helpful votes received in reviews: 90% (20 of 22)".
Systems using game mechanics
Users "unlock achievements" for their activity on the site. They can get specific statuses like "mayor", they also can receive different badges depending where they "check in" often.
Khan academy is an online learning site, where users receive points and badges for learning. So for example correct answer to X problems in a row gives you Y badge.
There are various types of badges, some are easy to gain for new users and some require significant efforts.
Stackexchange uses both vote points and badges.
"Reputation is a rough measurement of how much the community trusts you; it is earned by convincing your peers that you know what you’re talking about."
Reputation is used to specify what user can do on a site. Without reputation only basic functionality is available (asking questions, answering, suggesting edits), gaining reputation gives user additional "privileges".
Users vote on questions and answers, which gives following reputation:
answer is voted up +10
question is voted up +5
answer is accepted +15 (+2 to acceptor)
question is voted down -2
answer is voted down -2 (-1 to voter)
A maximum of 40 votes can be cast per user per day. User can earn a maximum of 200 reputation per day.
Users can also gain different badges when their reputation reaches certain level. Badges like "Editor", "Student", "Scholar" etc.
- Karma in Launchpad