With Dynamic404 you can redirect 404-pages to other URLs easily. This way, visitors will never get an annoying "404 Not Found" page anymore, but they will be redirected to the right page. Alternatively you can disable this redirection, and just enrich the 404-page with a list of possible matches for the current 404-error.
The Dynamic404 component determines which page is the closest match by looking at Menu-Items and articles to find a matching alias. Alternatively, you can configure redirects yourself from within the Joomla! Administrator. This way you can even fix URL-typos made by your visitors. Also, the Dynamic404 component can be extended with Joomla! plugins that allow you to search for matches within the logic of other third party extensions.
When good matches are found, you can determine yourself what to do: You can display an optimized 404-error page to your visitors which shows a listing of all the matches, plus links to your site search. Or you can transform the 404-error into a 301 or 307 redirect to direct the visitor to the right page.
Despite what some SEO-experts have claimed, Dynamic404 will give you a higher ranking when the automatic redirection of Dynamic404 is enabled: In case of a 404-error, that 404-status never reaches the visitor, but it is transformed into a valid redirect. This makes Dynamic404 vital when migrating between major Joomla! versions (Joomla! 1.5 to Joomla! 3 for example), or when migrating com_content to your favorite CCK.