I recently purchased a copy of the Zoom Search Engine to replace the existing site search engine that I was using on Hoax-Slayer.com. I have previously tried several site search engines, but I was not particularly happy with any of them.
However, Zoom is simply outstanding and is streets ahead of other search solutions I have tried. It is highly configurable, and allows webmasters to set up a very professional internal search engine that blends seamlessly with the design of your website. Most importantly, it produces reliable and relevant search results for users.
Zoom comes as a software package that installs on your computer rather than a fully web-based script and control panel like others I have tried. This means that you can configure your search, index your site and upload the necessary files from software installed right on your computer.
Moreover, the support offered by the company is excellent. After I bought the program, I encountered some quite server-specific problems with Zoom's logging function. Even though the problems were caused by the way my hosting company's servers are set up and were not explicitly caused by the Zoom software itself, Zoom's support staff went out of their way to help and even rolled out an updated version of the software that addressed the issues.
Zoom is free for small sites with other fee-based versions available depending on the specific needs of your website. I opted for the professional version and I consider it worth every cent and more.