Pick Block

Pick Block is a function that allows you quick access to blocks in your Creative inventory. It is present in Classic and in Beta (creative mode only). In Beta, it is possible to change Pick Block key.

Usage
Just point at the kind of block you want to select and press the pick-item key. The default pick item-key is the middle mouse button. The pick block function will now try to find the kind of block you pointed at in your inventory and select it into the selected slot.