Postica has to be one of the more single-serving Web apps I've seen in a long time. The service lets you create a slew of tiny sticky notes that can be maneuvered around the confines of your browser with ajaxian flair. They don't hover over pages you're looking at like Diigo or Fleck; instead it's all about your personal note space. Whatever notes you create are saved, and can be accessed from wherever. You can also share them with others, and they can send notes to your workspace, too.
Each note is confined to just 140 characters, the same length as an SMS text message or a note on Twitter. You can also add a single file to each note. I managed to get a few image files that were over 5MB in size, but it choked on the 50MB video file I tried. There's no documentation on what the size limit is, or if you'll run into any sort of cap on total storage so I'd stick to small files like PDFs, pictures, and office docs.
I'm still wary to recommend Postica over something like Shifd, a similar Web-based sticky note service that does a much better job integrating URLs, addresses, and letting you access and sync up your notes on both desktops and mobile phones. The one area where Postica has the leg up is file sharing, but you've got to be patient for each upload to make its way there.