On November 12, 2010 SearchBlox announced the availability of SearchBlox Search Software as a completely FREE product.
I took a look to SearchBlox on a complex project, and I suggest you to give it a try. It is a good solution for small sites, and it is powered by Lucene. Google Mini has a strong brand, but it cost a lot and it had a 2-year resubscription process: a very costly stuff.
Read here the complete story: SearchBlox is now available as a FREE product with no limitations.