Search tools for forum posts come and go. Lycos has its discussion search which focuses on numerous forums across the web, as well as Yahoo Groups and MSN Groups. Google Groups allows you to search Usenet discussions in addition to relatively newly created Google groups.
Omgili (Beta) is a search engine designed to index web-based discussion forums. Omgili's unique algorithm analyzes forums not as a simple web page, but as an active discussion with a title, topic and replies.