Who'da thunk the common mug could stand for so much improvement (or at least, variety)?
For the anal-retentive, the Tea-Code mug comes with a button to wind a teabag string around, keeping it out of your mouth when you sip.
The sliding rings on the Drink Selector Mug simplify your coffee order for that troublesome intern who keeps blathering about his degree from M.I.T., rather than focusing on what you're (not) paying him to do--fetch your joe correctly.
The Snap Cups can be affixed together by means of the integrated metal clothing snaps, though I'm not sure how easy these'd be to undo when filled with hot liquid.
The Carabiner Mug is presumably not meant for you to hang from while mountain climbing, but do provide a handy way to hang the mug off the back of your backpack, which will tempt your camping buddies to drop dead bugs into it whenever you're not looking.
All of these are from Beautiful Life's "25 Creative and Original Mugs" roundup, check 'em out.