Currently there are lots of discussions about search enhancements for SharePoint.
By far the most common and effective method of finding information is browsing a structured classification system (taxonomy). By default SharePoint has one taxonomy only: the site structure and navigation. You can enhance this using site categories and content types.
If you want to categorize single items or documents directly with centrally managed cross-site tree-style categories you can use taxonomy extensions, that are offered by several vendors, e.g.
http://www.sharepartxxl.com/products/taxonomy/default.aspx
The content categorizing can be done directly in the edit view of an item or document, guided by rule based suggestions or completely automatically. If the content is once categorized it can be found using additional navigations like category tree, A-Z index, tag clouds or related item links, directly shown in the items detail view, using default SharePoint search procedures in background.
Categories can offer different views to your content - independend from the items storage location, e.g. site or list or item type. This views can be based on the organizational structure, procucts and solutions your caompany offers, based on target groups or ordered geographically by language or subsidiaries. Category-based navigations can greatly improve knowledge bases, product or media libraries or intranets based on SharePoint.
Using categories your SharePoint portal really can become a place to share knowledge as well as content.