Mac: If you don't want to just leave your notes, to-dos and passwords sitting around, Bluenote is a Mac app that encrypts everything you put into it. The app locks the contents behind a password so nobody can see what you're up to.
Bluenote is a number of different things. At its core, it's a to-do and note-taking app that encrypts everything. It organises those notes and tasks in simple lists, and you can select from a huge set of icons so you can quickly spot what you're looking for.
The nice thing is that Bluenote automatically encrypts and saves everything you do, so you won't need to worry about saving or selecting security settings. Additionally, Bluenote has a password manager built in to store your passwords and offer suggestions to make better ones. It's not nearly as robust as something like Lastpass, but it's better than just leaving your passwords hanging out in Keychain.
Bluenote is also incredibly easy to use. Once you fire up the app, you're given a tutorial of the features, but it's hardly necessary. Adding tasks and notes is as simple as you'd expect, and you can organise them by just clicking and dragging. The interface is easy on the eyes, and you're never distracted finding out how to do something.
Syncing support is currently missing, but it's on its way in a future update. If you're looking for a way to hide your notes and tasks behind a encrypted password wall, Bluenote is worth a look.
Bluenote ($6.49 until April 5, $10.49 thereafter)