Open a character creation screen with F6 (F12 for 1.8) and fully customize your skin

You can change your model into any of the existing models within the game

Climbing only via gaps in the walls
Climbing ladders with different speeds depending on ladder coverage and/or neighbour blocks
Alternative animations for flying and falling
Climbing along ceilings and up vines
Jumping up & back while climbing
Configurable sneaking
Alternative swimming
Alternative diving
Alternative flying
Faster sprinting
Side & Back jumps
Charged jumps
Wall jumping
Head jumps
Crawling
Sliding

Hit HOME/END to open the Morph Selection GUI.
Hit ENTER/RETURN/LEFT CLICK to select a Morph.
Hit ESCAPE/RIGHT CLICK to close the GUI.
Hit DELETE/BACKSPACE to delete a Morph.

New versions use Square brackets (normally next to the Enter/Return key) to open the Morph Selection GUI.