There are a lot of notepad apps out there, and some of them have really cool functions. If you are craving simplicity, however, take a look at ZuluPad, a crisp, clean notepad application with a major hook.
ZuluPad allows you to create links in your documents that will allow you to jump back and forward between various related notes. The developer explains this well on his website: "You're a music major… studying Bach. You've taken notes on Bach in ZuluPad and (later) you're taking notes on Beethoven, and your teacher mentions a way in which Beethoven was influenced by Bach. As soon as you type "Bach" into your notes, it's underlined as a hyperlink. You think to yourself, "oh yeah, I know a few things about Bach". So you click the link, and you can read all of your Bach notes."
ZuluPad's actually a fairly simple notepad program, and if you are looking for even slightly advanced features, like the ability to insert images or basic formatting, you won't find them. The app does have some nice syncing functions, however, which mean that you'll be able to access your notes from multiple computers without transferring files. Just bear in mind that this feature requires you to register.
ZuluPad is a basic notepad app with added syncing and in-document hyperlinking.