I played with 'sphider' today and it does work, but needs tweaking, it's quite old: http://www.sphider.eu/
Best bit is the ability to search using more than one word and one install can serve several sites.
Quite complex and behind with new database protocols. Will need updating. I'm sure the author won't mind if it improves it.
If you do try it you'll need to change '14' to '6' on line 139 of install.php. Throws error if left to 14.
Searching for multiple words is supported. A word can be excluded by putting the "-" character before it. Phrase search can be used by putting the search term inside "" characters.
Search is in an iframe. Target is parent. Added target to line 75 in search_results.php.
I'd love to see an option for 'advanced search' in a core block, and I'd love to see it used on the c5 web site: finding forum posts or documentation pages using multiple terms is not fun ATM.