Script Libraries
There are a number of sites that have a selection of pre-built Javascripts available for you to copy and use on your site. Some are better than others. These reviews will help you to work out which libraries are worth using.
Hot Scripts
Hot Scripts provides a range of scripts that you will not find listed together anywhere else.
Dynamic Drive
Dynamic Drive provides a reasonable selection of scripts.
Javascript Source
Javascript Source provides one of the largest selections of Javascripts anywhere on the web.
EarthWeb
EarthWeb also provide a large selection of Javascripts.
Javascript Kit
Javascript Kit is a reasonable resource for Javascript tutorials but the library of scripts on the site could be better.