Reeder for Mac - If you have unread news items, the dock icon looks like box filled with papers. When you have nothing to read, it looks like an empty box.
Wunderlist - The desktop version has one form for both signup and login.
/via Denise Pires
Mac OS X - Pressing Alt ⌥ and clicking the volume icon lets you select input and output device.
Alfred - Highlights the keyboard-shortcut when you disable all other GUI to access the preferences.
/via Hans Petter Eikemo
Mac App Store - The progress bar of installing an app from the Mac App Store changes its dimension separate from the icon, when magnified in the dock.
Cornerstone - The number of segments in the spinning activity indicator equals to the number of running activities.
/via Yusuf Miles