Windows/Mac/Linux: Portable, cross-platform note-taking application KeepNote stores your notes, thoughts, outlines and more in a simple hierarchical format, complete with rich-text formatting, images, attachments and even plugins.
The application itself is simple enough to use: just add "Pages" on the left-hand pane, and then write or paste anything into the note itself — but you can organise the tree menu with icons, folders and add new nodes easily from the keyboard.
Since the notes are each stored in a separate folder on the filesystem, in HTML format, you can use a file syncing application like Dropbox combined with the portable version of KeepNote to use your notes from any PC. KeepNote is a free download for all platforms. Thanks, Blaine!