Recs.
Updated
Uses Spotlight's search engine but offers a better interface for searching and listing results.
Specs
Pros
Pro Interactively refine search
You can add criteria to reduce the number of search results. “Find by example” allows you to use drag-and-drop to find or exclude similar files.
Pro Highlighted preview of your searched text WITHIN the file
For those who need to reference quite a bit, it makes it so much easier to find specific topics or subjects with 1 or 2 key-word search and be able to not only see exactly where it's saved, but to be able to preview your search highlighted from inside the file.
Cons
Con Can’t search network drives
HoudahSpot doesn’t build its own index. It relies on Spotlight so it can only search what Spotlight indexes, which does not include network drives.
Update: While this is true, advanced users can enable Spotlight indexing for a network drive by using the following command in the terminal: mdutil /Volumes/name -i on