Synthesis Studios has released version 2.1 of FileSpot (formerly MoRU), their advanced interface for Spotlight. It allows you to make advanced, boolean logic queries and makes accessible some of the more complex aspects of Spotlight. Not to make it sound complicated, though, its iTunes-ish interface is pretty simple to use. It also adds file tagging with support for other 3rd party tagging apps like Spotmeta.
This release adds two great features: Quick Look support and search results that display as they're located rather than waiting for the search to complete. Both features definitely make my day.
FileSpot has a 30 day trial period and costs $20 to register.