Note: The creative mode is still work-in-progress, in other words, it does not contain all features yet, and there might still be technical issues. It's recommendable to use the creative mode in singleplayer only, or when using it in multiplayer, just for a certain group of people 
The new creative mode was introduced with update 0.6.3 Alpha. The first release only contains terrain tools, which can be accessed by pressing F5 (when you are in creative mode, either by setting the parameter during world creation, or by using the "setgametype 1" command).
Pressing F4 enables the "semi-creative mode", i.e. no special tools, just instant digging and godmode. But in this mode, you also have access to your items and your inventory (which are disabled when you're in terrainedit mode). You are also able to craft items for free at the workbenches (depending on your permissions).
When you are in terrain edit mode, you see a red square in front of you. That's the area your actions will affect. You can resize it by pressing + and -.
You will notice that you have three tools available: Add/Subtract, Smooth and Paint. Below your tools you can see a material selector (can be changed by using your mousewheel).
- Add/Subtract: Left mousebutton adds terrain, right mousebutton removes terrain. You can also press your Return key ("selector modular") to enable the "flattening mode": You will see a "limitation", and as long as it's active, you cannot fill terrain above this border or remove terrain below this border. Very useful for creating flat areas or plateaus. You can change the level of the flattening border by pressing PageUp/PageDown
- Smooth: Basically the same as the rake does in regular mode, but without turning everything into dirt automatically, and also with no limitation (works on all materials). Very useful to fix broken chunk transitions, or to create nice hills or mountains after using the Add/Subtract tool
- Paint: This tool "paints" the currently selected material onto the terrain. So no terrain will be added or removed, just the existing terrain will be replaced by the new material. Nice for turning large dirt areas into grass again, or vice versa. Also quite useful for creating paths of dirt or gravel
As usual you can use the flymode (F2) and the magical light (L).
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Add/Subtract and Paint tool in action:
Flattening mode enabled and Plateau created easily with the flattening mode:
A strange sculpture and a nice man-made cave (just took 2 minutes 
