WordPress already comes with a decent search widget you can take advantage of to add search capability to your website. It is pretty standard and nothing fancy. You may get a fancy search box if you invest in a decent premium theme. WP Ultimate Search could help too. It is an attractive search plugin that adds AJAX search and filtering ability to your site.
WP Ultimate Search searches post title and body content. You can search by multiple keywords or full phrases. The script highlights search terms in results. You can filter results by category. It is AJAXified and quite sleek. WP Ultimate Search is lightweight too. It bypasses WordPress’ built-in search functions to lower the load on your database. The plugin has potential.