One of my clients wanted the ability to limit search to within a category. I found this great code on WebsTutorial
This code gives the ability to search over two categories at the same time. I just removed the second category field and it seems to work great. All without a plugin. Great piece of functionality that we can now offer to our clients!
You also need to include PHP code functionality in widgets if you want to have this in a widget. I found this code at this site: http://www.emanueleferonato.com/2011/04/11/executing-php-inside-a-wordpress-widget-without-any-plugin/
You need to put this code into your functions.php file: