Search Fixer makes «pretty» search links work properly. A pretty search link usually looks like this: http://example.com/search/waldo Because of a bug in WordPress, pretty search links with spaces in them do not work. Search Fixer fixes that bug.
If you use Search Meter‘s widgets, you should install Search Fixer too.
Technical details
The bug is WordPress bug 13961. This prevents «pretty» search URLs from working properly. For example, http://example.com/search/hello%20world should search the example.com blog for the words «hello» and «world», but because of the bug it actually searches for «hello%20world» and fails to find anything.
When the WordPress bug is fixed (probably sometime in 2011) Search Fixer will no longer be necessary. I will keep Search Fixer up to date so it won’t interfere when the WordPress bug gets fixed.