FreeMC is a NativeClient(NaCl) implementation of MineCraft.
Currently only a few features are implemented but it is looking good.
Check out an "older" live demo here (login as demo/demo).

Keyboard Controls (similar to the offical game)
AWSD / Arrow KeysMove player
EView personal inventory
Left ClickDig
Right ClickPlace/Use block
1-9Select active inventory slot
ENTERSend a message (chat)
/Enter a special command (tp, gamemode, give, clear)

TToggle creative/edit mode (if server allows)
GToggle gravity (must be in edit mode)
CToggle clipping (walk thru walls) (must be in edit mode)
RFly up
FFly down

Project Page Download