Chrome: Half of the time, I don't really remember why I left a tab open. I have dozens of tabs meant as references for later, but I'm constantly forgetting their original purpose. With Note Anywhere, you can leave a little reminder for yourself on any page. This Chrome extension is very simple and self-explanatory. It really just lets you add virtual Post-it notes anywhere in Chrome. (The only pages it might not work on are web apps that are always monitoring your keyboard for keystrokes, like Google or Tweetdeck.) You can even close the tabs and the notes will be there when you navigate to the same page again, although I wouldn't use it to store important information, of course.
You can customise the colour and font of your notes, but otherwise it's a pretty bare-bones extension with a coded-by-hand aesthetic. And, just like sticky notes in real life, its simplicity is its virtue.