I could be wrong, but I have been under the impression Google set that as a standard of sorts years ago, starting with GMail, and continuing with their other products. Pivotal Tracker has done it for a while, too. I think there are others, but I cannot recall what they are.

I press '?' on any site that I think might/should have keyboard shortcuts!

And Google news, docs, calendar. Not the Google search pages though, since key presses are intercepted by the search bar.

The new Google Groups also has it. The shortcuts are similar to Gmail's.

Works in Yammer too (I work there)

Twitter as well.

Personally, I dislike websites that try to grab my keys. I use pentadactyl/vrome, so if the website just lists everything I need to do there as a normal hyperlink, I need only hit f+number and I'm good to go. That isn't to say that it isn't innovative or cool. It's just not up my alley. I like to let local applications manage the keyboard.

I agree that websites should not be able to override your keys. I'm often annoyed when I press '/' in firefox, expecting quicksearch to come up, and instead the website hijacks it for their own search.

However, I don't think that this should be solved at a website level. Websites shouldn't know or care what keys do in your browser; they should simply provide reasonable shortcut keys. Browsers should make it so that their shortcut keys cannot be overridden without your explicit permission.

This is mostly the fault of insufficient Chrome addon API afaik. In Firefox/Pentadactyl the addon automatically hijacks all the keypresses and only by pressing CTRL-Z can you fall back into direct mode and feed keypress events to Javascript.