The shortcut keys in Gmail are great but there is one that is missing: Delete. Yes, yes, you are never supposed to delete anything in Gmail and there are shortcuts for reporting spam and archiving. Nonetheless, some users really want to delete stuff and a few of those users would like to be able to do it without clicking the delete button.
Gmail Macros adds additional keyboard shortcuts to Gmail. Some are obvious (and have been done by other scripts) such as “t” for move to trash and “r” for mark as read. However, I strove to provide a bit more functionality.
Gmail Macros is useful indeed. There are also a couple of other useful Gmail related greasemonkey scripts in the original post that are worth checking out.