Just in case you don't know, you can create or delete a keyframe at the current time for a property by hitting option(alt)-shift-and the properties shortcut, ie p for position:
Some examples:
option-shift-A add or delete anchor point keyframe
option-shift-P add or delete position keyframe
option-shift-R add or delete rotation keyframe
option-shift-T add or delete opacity keyframe
etc.
There's still a few properties which I don't think are accessible by direct shortcuts, ie time remap keyframes, effects, etc.
Kyle
And I don't think a script would be much use for this. It isn't possible to trigger a script by pressing a key. You could create a panel with an "Add Keyframe" button (that adds a keyframe to the selected property when clicked) but this is no better than clicking in the Timeline to add a key.