I love the free web sticky notes available at the mystickies.com site. I made a screenshot so that you can see what they look like.
Even though I can put them on any page I want, you wouldn't be able to see them unless you could view my monitor or look at this screenshot.
Rather than just bookmarking a page, now you can sticky note the page with a message as to why you bookmarked it in the first place.
The only thing I don't like is that I usually place my non-virtual stickies at a bit of an angle and these have to be perfectly straight.