Windows/Mac/Linux: While it didn't make the top five note taking apps, if you're looking for a utility that's more powerful than a text editor but lighter on the system than one of the more robust web-enabled utilities in the roundup, consider Tomboy, a free, open-source note taking utility that supports Windows, Mac OS, and Linux. Tomboy supports linked notes, easy editing and quick search, and even syncing notes to Ubuntu One so they're accessible on other systems.
Tomboy has some other OS-specific features. For example, in Windows 7, you can pin Tomboy to the taskbar and see recent and pinned notes by right-clicking it, or search all notes right from the taskbar. In all cases, you can link your notes together, connect your ideas, and open and edit notes quickly.
The app also has an active development community around it. Contributors write and update plug-ins that allow you to host your notes on your own server, for example, leverage Dropbox, or sync securely over SSL. Tomboy is one of those applications that can be simple and lightweight if that's all you need, or feature rich and full of added tools if you're willing to dig into it.
Thanks to Ray for sending in the tip!
Tomboy [Gnome Projects]