From the blog "Sandbox Update Notes"

+ Added swimming + Added day/night cycle + Added rain (can be toggled by pressing F5) + Added support for world loading/saving (some things do not save yet, however most things will save as expected) + Added world selection screen + Added world information screen * F3 screen now shows viewport resolution * F3 screen now shows the tile selected under the mouse cursor * F3 screen now displays the current world time * Tweaked soccer ball physics * Dropped items now have animatio