JavaScript
JavaScript Marquee v3
The latest version of this ever-popular cross-browser scroller script including features such as diagonal scrolling and wipe transitions. Actually, this hasn't been updated for several years and is starting to lose compatibility. So far it has broken in Opera 9.
Status: v3.0 Updated November 2003
Greasemonkey User Scripts
These are User Scripts for use with the Greasemonkey extension for Firefox and can also be used with Opera.
User Scripts allow you to rewrite web pages once they've loaded into a browser. This can be used to re-arrange content, remove annoying features or even pull material from one site to enhance another.
Perl & PHP
LJ Fortune
This is a little server-side toy that uses LiveJournal's RSS feeds to produce results. This one is a simple word association analyser that guesses a user's future in various areas based on their last twenty LJ posts. It is a combination of a PHP front end piping results processed by a Perl agent (which is far more elegant in handling large data sets). The results are designed to be cut-and-pasted to users' journals.
Words in Collision
...or When Words Collide, or War of the Words, I couldn't decide. This toy simulates a word game played between two LJ users. Like LJ Fortune it uses a PHP interface and a Perl agent to process data collected from two LJ RSS feeds. In this case it is not simply a matter of elegance but of Perl's raw power in comparison to PHP.