wpSearchMu is a very powerful search plugin for WordPress Mu. It is based on the open source search engine "Lucene" which means its fast and
I have been trying to find a decent multisite search option and although this plugin is reported as no longer updated, I thought I'd give it a go - just in case.
Sadly though, the indexing doesn't seem to work - it only indexed one of my two blogs. This is a real shame as this plugin, if it could be restored to it's former glory, looks like it works well otherwise.