Providing members and other visitors to your site with the capability to search the site can be very valuable ... especially if you have lots of content on the site.

If your web site is extensively indexed by Google, then that opens the door for a good and inexpensive option.  Google has a free search tool that can be added to your site. (For an example, see its application on AXI's site -- http://www.axi.ca/search.html)