https://codecanyon.net – One of the best scripts directories out there! Right now it has over 17,000 scripts and plugins, and their prices start at just $1
http://www.best-php-scripts.com – Best PHP Scripts is one of the best free scripts directories. It includes a few paid scripts as well, but most of them are 100% free and open source.
http://www.phpfreecode.com – Php Free Code is another website that provides lots of free scripts. Categories include social sites scripts, seo scripts, image gallery scripts and much more.
http://www.hotscripts.com – This website has over 45,000 scripts and code snippets that are listed in over 1,000 categories. Php mailers, Facebook scripts, shopping carts, etc are all in here. Some of the scripts are free, and some paid.