Hitting '?' at anytime during your use of Google Reader will display this, what I think is an, incredible HUD for Reader's keyboard shortcuts.
Google Reader's Heads Up Display for Keyboard Shortcuts
When I saw this I immediately thought: "Wow, Google really gets this whole web application thing.". But what else can we learn from this? I think there is definitely something to be said for quickly explaining the keyboard shortcuts in a way that is available throughout the entire application, at any time, with one keyboard shortcut.
Seeing this in Google Reader has prompted me to get my butt in gear and begin to, at the very least, document these features that Viddler has so that people can find them.