If your content is dynamic (database driven), you could easily whip up your own search interface in a few minutes. If it isn't, you'll need to rely on some technology that "scrapes" content from pages, which is a lot tougher.
Yet another huge plus for dynamic content. If you are looking for an easy way to move your static pages into a dynamic backend, I'd recommend using WordPress. Not only will you gain the ability to utilize the included search capabilities within your templates, but you also gain a nice little CMS.
If your content is already dynamic, is it housed in an SQL backend? If so, I can give you some ideas as to how to generate search queries.