Though the ideas may not be original, the code and logic is entirely mine unless stated otherwise. I realize there are many other versions of a few of these things, but don't copy mine without permission. 

Userscripts

Userscript are javascript that runs every time you load a certain webpage, if you have Greasemonkey for Firefox. Links will be to pages on Userscript.org which host said scripts. 

• Flyne's Autofill

A userscript which allows you to save multiple usernames and passwords for a single login form.

•  Cup of Tea

 A userscript to load different webpages in new tabs depending on the day. Probably better suited for an extension, but I've never seen one to do it and I can't make one.

• Comic Archiver
  

A userscript which saves the URL of the biggest image on each page you visit for later retrieval or download with something like DownloadTheMall. I mean, DownloadThemAll. Mainly to save entire webcomics, but can be used with other stuff.

• Delicious Links 

A userscript to prevent re-bookmarking things on del.icio.us. I don't actually use it, because I don't use del.icio.us. Supposedly it works, though. 

HTML 

Webpages containing javascript which does cool stuff. Links are to the pages.

•  File Saver

A webpage using a firefox-specific capability which saves URLs to your local hard-drive, if you give it permission.

• DeviantArt Link Generator 

 Another firefox-specific page which fetches links to every image of an artist on DeviantArt.com. You must be logged in and have it set to full-view by default for this to work. Also, you have to grant it permission, and it will ask many, many times, so I recommend allowing permanently. All it needs it for is to get and parse the HTML, though.

Bookmarklets

Bookmarlets are bits of javascript to do one specific thing, often on a specific webpage. To use, right-click on the link and bookmark it, then navigate to the page it runs on and open the bookmark. Links are to the javascript itself, and just clicking on them is not recommend.

• Zoom Google Map

Zooms Google maps (maps.google.com) in further than they normally can. Works only on Google maps, and even then only in certain locations. Try the Google campus.

Canvas

Stuff I've done with a wonderful thing called canvas, implemented only in browsers like Firefox or Camino (and to some extent Opera and Safari). It's basically a way of drawing stuff in a webpage. It's hard to explain more precisely, but the pages linked to here should give some idea.

• Sierpinksi Triangle Generator
  
• von Koch Snowflake Generator
• Horizon Generator
• Line Connector 1 
• Line Connector 2

    Simple Mandelbrot Generator

A very simple Mandelbrot set generator. May take some time to draw.

    Mandelbrot Generator

A not-simple Mandelbrot generator. Accepts user input and zooms and stuff.