As a web programmer, I love to hack web pages that does not fit my needs, example:

Google Reader on a UMPC:
The UI of GoogleReader was way too complex to be displayed on a 7inch UMPC.

Many forums/discussion groups:
Many of these forums developed code to prevent extra bandwidth loading on IMGs, so javascript was used to do so, example, users have to click on the IMG to load it.

Many download sites:
I hate it on how some site developed stupid code for user to download a file, example, count down on time counter, funny/stupid javascript calls to hide/dynamic gen URL, all I want is a direct link to the file….

Here comes the cure, GreaseMonkey Addon, which allows developer to change how pages are render, or even add functions and automation.

If you are interested in learning it, this site is a great start.
– Dive Into GreaseMonkey
http://diveintogreasemonkey.org