Google Mail has extensive keyboard shortcuts available - for example, when you are reading an email you can just press "r" on the keyboard to reply to the message (or "a" to reply all, or "f" to forward) - you can also do things like archive the message you are looking at and go to the next message just with one keystroke. You probably won't use
all the keyboard shortcuts, but using just a subset can really increase productivity.
To enable keyboard shortcuts go to settings (the gear icon towards the top right, then Settings) and in the general tab you'll see the option to enable. Once enabled, you can get a full list of the available keyboard shortcuts when you are in Google Mail by pressing the
question mark key on your keyboard (that is, press shift+?).
More information from Google at http://support.google.com/mail/bin/answer.py?hl=en&answer=6594