Google Moderator graduated from a side-project of Google's not so long ago after being adopted by a number of high-profile users including the White House. It enables moderators to receive questions that are voted on by the community in order to answer the most popular ones.

Yahoo! Answers is one of the most popular Q&A sites on the web. It provides an open, unmoderated format that allows users to ask questions to which the community can provide answers and vote on the best one.

UserVoice connects customers to their favorite products and services by providing a robust set of feedback and recommendation tools. Customers are allotted a certain number of points to use for submitting new ideas and voting on feedback from other users.

Aardvark was recently acquired by Google due to its ability to connect users through email, the web, or instant messaging. When you ask Vark a question, it finds the right user to answer it based on their interests.

SuperUser is part of the StackExchange network which includes a nearly unlimited number of Q&A sites on a wide variety of subjects. The code behind each of these sites is open source and freely available to the public.

BlurtIt is a relatively new player on the scene which provides many of the services offered by the standard Q&A sites such as Yahoo! Answers. The difference is in the experience, which is much more user-friendly than most of its competitors.

KGB primarily serves the mobile community by providing answers on-the-go. While the services provided are premium, they do promise professional results from their experienced KGB agents.

http://www.quora.com/

MakeFive delivers a twist on the typical Q&A site by providing users with the ability to create their own Top Five lists. Each list is loaded with a variety of social networking tools which allow users to add their own item to the list and vote on their favorites.