Available Keyboard Shortcuts for Web Applications
If you need to know only standard browser keyboard shortcuts, they are listed here.
Separate browser's shortcuts lists:
Internet Explorer's shortcuts can be found here.
Firefox's here. (scroll down a bit)
Google Chrome's here.
Safari's official source here, but a better one is here.
Opera's here.
And I find this comparison table ugly, but very useful.
If there is no input focus, you should use plain keys, e.g. a, s, d, etc.
Otherwise, here is a list of open keyboard shortcuts (at time of writing) mostly derived from Chrome (OSX), with additional eliminations made from F. David McRitchie's Comparison Table. Thanks to @leonbloy and @KnowYourElements for Firefox shortcuts.
This list does not consider Alt/Option since they are less familiar and often less ergonomic for users, but most of them are open.
As a cross-browser list is effectively a logical AND of browser-specific lists, the below list actually narrows down the search quite a bit.
// (Ctrl ≅ ⌘ on OSX)
Ctrl + I
Ctrl + U
Ctrl + Shift + F
Ctrl + Shift + L
Ctrl + Shift + O
Ctrl + Shift + S
Ctrl + Shift + U
Ctrl + Shift + Y
Please comment if any of these are not available on Windows or other browsers, and I will edit the list accordingly so it is most useful.