www.yahoo.com/docs/yahootogo/index.html
http://www.freefind.com/

Searches fall into two categories - letting your site visitor search the Web, and letting your visitor search your site. The first are simply front-ends to the major search engines, such as Yahoo-to-Go (www.yahoo.com/docs/yahootogo/index.html). Your visitors type queries into a small on-page form, and are automatically taken to the search engine's results page. It's a nice enough feature, but not a major addition to your site's functionality.

Far more interesting is the idea of responding to visitors' queries with links to relevant pages in your own site. To set this up, you register your site with a third-party search engine, which sends a 'spider' applet to look at your pages. It then builds an index to your site, which it stores on its own computer. When someone enters a query, it's passed to the search engine, which checks its index and returns the results.

You have to notify the search engine of any changes to your site, otherwise its index will become out of date. However this isn't particularly arduous, as you don't need to specify the exact details, merely ask it to send its spider on another visit. I've indexed my site at http://www.freefind.com/ - it's easy and free, and doesn't demand any advertising on your site's pages (although the search results pages, do, of course, contain the ubiquitous banners).