You have Gravity enabled - press Tab while you are holding Right Mouse to turn Gravity off.
While you are holding Right Mouse all the available options are listed in the Status Bar at the bottom of the screen - about 3/4 of the way along you will see 'Toggle Gravity' next to a symbol representing the Tab key.
Check out some of the other options like - Shift key for faster movement, Mouse Wheel to speed up and down, or Spacebar to teleport onto surfaces.
Right Mouse Navigation
i cant figure it out? when i press the right mouse button i just drop straight down under the grid. thats all i can make it do? what am i dont wrong?