mgsimon

Pretty Permalinks requires outofthebox the module mod_rewrite. This module is a workaround to use pretty permalinks without any rewrite rule. The errorhandling – customized errorpage – will be used to resolve pretty permalinks. mg404Rewrite-Modul sends HTTP-Header-Code depending on the request. 
If you have any posts to permalinks, you can try to activate PostProxy to resolve this issue (configurations / misc).
<h3>Licence</h3>
GPL (http://www.gnu.org/copyleft/gpl.html)

mg404Rewrite

This module is a workaround to use pretty permalinks without any rewrite rule.

craigsimps

This is a super-simple plugin which will turn WordPress’ default <code>/?s=query%20terms</code> into <code>/search/query+terms/</code> if you have pretty permalinks enabled.
This is something which I have been placing into a theme include or in a core functionality plugin until recently, but it makes sense to separate this functionality out into an individual plugin so it can be switched on and off independently.
The plugin has no administrative interface or control panel. Activation turns it on, deactivation turns it off. Once activated, performing a search using the standard WP search widget should give you a much nicer search URL.

Simple Search Rewrite

Redirects search results from /?s=query to /search/query/, and converts %20 to +.

mg404Rewrite vs Simple Search Rewrite