Chrome: Google Keep isn't the most full-featured note syncing service around, but it works well for casual note taking, and an unofficial Chrome extension lets you access it extremely quickly.
After you install the extension, navigate to chrome://flags and enable the "Enable Panels Mac, Windows, Linux, Chrome OS" option. Now, just hit the extension's icon at any point to bring up a Gmail compose-style panel at the bottom right of your screen. From this panel (which you can resize in the settings), you can add a new note or to-do item, and view or delete the rest of the items already stored in Keep. It's so simple, that's it's surprising Google didn't make the same thing itself, but now at least you have the option to use Keep without monopolising an entire browser tab.