Relevanssi is a plugin for WordPress that replaces the existing search engine with one that does a better job of finding relevant results and is far more configurable than the default search. I especially like the fact that it will not only let you decide whether or not to index comments or user profiles, but also let you assign weights to content types for determining relevancy.
Relevanssi is available in both a free version from WordPress.org and a premium version available from Relevanssi.com The premium version comes in several different licensing flavors — for $9.95, you can install it on an unlimited number of personal blogs, though with no support. There are also standard and developer licenses at $39.95 and $69.95 per year that add additional support and installation options.