Microsoft is nearing release of Enterprise Search Server 2008 Express software. The Release Canidate is a FREE download that is a available now. The RTM version will be a free download as well. 

Windows Server 2003 is the required platform for installation of Search Server 2008 Express. The Express edition has limited scalability - where it supports only one “search application server” but support an unlimited number of “web front-end servers”.

You can see a complete list of the extensive features on the Microsoft.com/EnterpriseSearch site. Try it out.