Mamute!

Stack Exchange is a fast-growing network of 84 [and counting] question and answer sites on diverse topics from software programming to cooking to photography and gaming. We are an expert knowledge exchange: a place where physics researchers can ask each other about quantum entanglement, computer programmers can ask about JavaScript date formats, and photographers can share knowledge about taking great pictures in the snow.

Askbot is a Q&A system, like StackOverflow, Yahoo Answers and some others. Askbot is based on code of CNPROG, originally created by Mike Chen and Sailing Cai and some code written for OSQA.

Askalot is a next generation CQA (Community Question and Answer) system.

BitAsker is a paid question and answer plateform using Bitcoin ... a user can win money by answering a question.

LampCMS is an full-featured Open Source Question and Answers web program written in PHP with deep integration with Facebook, Twitter, Google, Linked-IN, Tumblr and Blogger APIs