Hot Scripts has an article on the top 10 simplest and most lightweight scripts on the Web.  The scripts include:

1. simpleCart(js) – A Lightweight JavaScript Shopping Cart
2. GetSimple CMS -The Simplest Content Management System EVER
3. FluxBB – A fast, light, user-friendly forum application
4. Relay – AJAX Directory Manager
5. Task Jr – Web-based task manager
6. IMAGIN – Flash Photo Gallery 
7.  Shoutbox – A PHP + AJAX Shoutbox
8. lwrte -Lightweight Rich Text Editor
9. Adminer – Database Management in a single file
10. EZPX – Photoblog System

 Does anyone have experience with any of these scripts?  You can read the article here.