It's a combination Firefox extension (or bookmarklet if you use another browser) and web service that works very seamlessly to help you annotate the web. To create a new sticky note, you just hold down Alt and drag with the mouse anywhere on the page. If you highlight some text first, the text will be automatically copied to the sticky note. Once created, notes can be resized or moved by dragging, and you can also add tags to your sticky notes. Once you've made some notes you can log in to MyStickies.com and see all the notes you've made and view them by tag or by site. There's also a "Sticky Page" feature that creates a blank page on MyStickies.com just for your notes.