The things you can do with JavaScript are pretty amazing. You can stop people from submitting blank forms or from coming to your site from unspecified pages...you can redirect visitors based on info they give you...create rollover effects and other special effects easily...password protect your site...collect stats on your visitors...and more.

Trying to list it all would be ridiculous. Let's get down to business.
The categories are up above. Search for what you want or check out a category and look around. Cut & Paste. Instructions are kindly included with all scripts requiring any tinkering.

Please note that most of these scripts were from one of these two sources:
http://javascripts.earthweb.com
http://www.javascripts.com

This collection of scripts at slackerhtml.tripod.com is merely a filtering of those two archives into a smaller and more selective archive, including only scripts that function on both browsers, and attempting to list as few duplicate or very similiar scripts as possible.