I've got one, but the free version only indexes 50 pages, so you have to pay more for more results. However, I imagine you can probably find a much better solution at
www.hotscripts.com and search for 'site search' scripts. If you're on wordpress, I'm sure there's going to be a free plugin.
In general though, I don't really find too many people using these search features at all, so I wouldn't fret about them being a neccesity. Although they do look quite flashy and make your site look good I spose.