Hmm..are you talking about for your forum for users to search throughout your forum topics and posts, or are you wanting to search for forums on the net?
Here ya go: http://discussion.lycos.com/ It's a pretty neat tool, but I don't think many people know about it.
Well all search sites technically infringe upon some laws but they are a useful resource so are often overlooked.
Nice tool Sharpseo. Thanks! Here's another one. You can search up to 15 forums simultaneously. Type in "bot surfer" into your search engine and download this free software. You can input your favorite 15 forums into the software, type in a keyword related to what you're looking for and find related posts. Saves a lot of time searching one forum at a time.
http://www.boardreader.com The results look like it hasn't been updated since the 22nd of November, but if you're looking for older topics it works fine...