Mac: Sticky notes are a common way to keep notes, and Ghostnote takes that idea to the next level. Ghostnote allows you to attach notes directly to applications, folders, files and websites.
Ghostnote is more about short-term notes than long term ones, so it's best to think of it like a sticky note opposed to something like Simplenote. Essentially, when you attach a note to something, Ghostnote remembers the context so you can quickly return to it. So, if you're working on a web site, you can attach a note to it reminding you to work on something specific. Or, if you're working on a project in a folder, you can do the same. It's amazingly useful once you get the hang of it. Better still, you can check out a trial to see if it works for you before dropping the $US9.99 on the full version.