Hi! I just launched my search app for Mail.app today. I've been working my butt off on it for almost 9 months during off hours (I have a day job right now).
It is a Apple Mail plugin that takes over the search field and provides a better engine. It does stemming, contact auto-suggestion, has powerful filters, and supports a large range of syntax.
I'd love to have HN take a look at my site and product, and let me know how I can make it better.
http://www.getrocketbox.com
Great looking site btw, although I've never had a problem with searching for mail with spotlight / Mail.app and nothing I see here convinces me of the benefits. Maybe more obvious and cool screenshots?