http://www.keyvan1.com/package/features/askExpert.asp http://www.alstrasoft.com/askme.htm Those are two that I know of. If you have enough programming / development knowledge to work with an Open-Source platform, why not just start from scratch and make your own? Unless your pressed for time, of course. I don't know of any free scripts though. Sorry.
Scriptowl.com has an answers search engine. Works like a charm imo. You should go check it out, as it requires a hell lot less maintenance, and you don't need any content, as it pulls it directly from yahoo.
No problem. I would use it on one of my sites, but I don't have the time to promote such site. Good luck on your side, though.