Microsoft has released Microsoft Search Server (MSS) 2008 and MSS Express. Both of these server products are free enterprise search products that are useful in scenarios where a full installation of MOSS is overkill -- you just want to have the enterprise search capability.

Info about the new products is here:

http://www.microsoft.com/enterprisesearch/products.aspx

And MSS Express is free. Get it here:

http://www.microsoft.com/enterprisesearch/serverproducts/searchserverexpress/download.aspx

Updated to make clear that only MSS Express is free.