Mac OS X only: Heavy iCal users who want to add a new event or new task at the press of a key combination want FlexCal. The System Preferences pane lets you set a hotkey that invokes a new to-do or new event entry prompt. Type in the details, press Enter, and FlexCal adds the item to iCal without launching the program. Here's what the entry form and preferences pane look like.
Above I've set Opt+T to invoke a new To-Do entry pane in FlexCal, which looks like this:
FlexCal is a free download for Mac OS X only.