When the user pushes the ? key, QuestionMark.js triggers a modal window that displays keyboard shortcuts for your app — similar to what happens on Twitter, Gmail, GitHub, etc. Hat tip to Robert Nyman. The modal is removed when the ESC key is pushed or the user clicks behind the modal.
I think every app that has keyboard shortcuts should do this. This could also be a generic 'help' menu for the app, but that would require some tweaking to the code and the CSS. For now, this works primarily as a shortcut keys menu.
To make it work, it's pretty simple. Link the CSS file in the <head> and run the script on page load (or at the bottom). Make sure all the mandatory files are included (question.mark.html, question.mark.css, and question.mark.js).
To customize, edit the question.mark.html file to include your own keyboard shortcuts. For each <ul> included with a class of help-list, a new column will be created. If you want a single column, use a single <ul>. Longer columns will scroll vertically.
Within each <ul> a single key/definition combo is inside one <li>, within which there is more markup. Edit the text to include your own app's key combos. It should be pretty self-explanatory when you look at the example markup.
If you have only one column of keyboard shortcuts, then you should remove the onwindowresize section of code.
The script is about 2KB minified and gzip'd and it has no dependencies. It's also more or less responsive (doing this via JavaScript).
The content from question.mark.html (which also holds all the markup that builds the modal) is loaded via Ajax and inserted into the <body> element of the page. The modal starts out invisible and is displayed via CSS transitions. Browsers that don't support transitions will display it instantly with no transition.
The script also uses window.onresize to manage width/height of the modal, which can have performance issues. If you find this is slowing down your app, just comment out the part of the code that looks for the window resize. The drawback to this is that it will display at full size on a small window. Of course, if you only have a small single column of keyboard shortcuts, you won't need the window.onresize part, so, as mentioned above, just remove it.
This should work in all the normal browsers and IE8+. No IE6/7 support. If you want IE8 support, include attachevent.js before question.mark.js.
The script is pretty new and I'm sure there will be reported issues and whatnot, so feel free to fork it and make it better. I don't know of any other scripts like this one, so you can help optimize the code and make this the go-to standard for including a shortcut key menu in web apps!
For a demo of QuestionMark.js, go here