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