I discovered the Firefox extension Greasemonkey in early March 2005. I had probably heard of it before then, but I don't know why it had taken so long for me to really look at it. It's exactly the kind of thing that appeals to me: injecting Javascript (often with DOM actions, possibly with Ajax) into any web page. Every once in a while, I come up with an idea for a one-off script. They're all available from my profile on Greasemonkeyed.com.