The directional light settings in the Map Editor can make some really nice effects, but they're also quite hard to figure out. I was adding some tips about angle values to the Map Editor for v1.2 and thought I'd post the "long" version here.
The light source rotates on the Y and X axis (see Picture). So:
Y (controls 'left to right'):
------------------------
0 = Shines North
90 = Shines West
180 = Shines South
270 = Shines East
45 = Shines North West
135 = Shines South West
225 = Shines South East
315 = Shines North East
X (Controls 'up and down'):
-----------------------
Important angles
0 = Shines Level
45 = Shines Diagonally Upward
315 = Shines Diagonally Downward
Other Angles (I recommend you ignore these 
)
90 = Shines Straight Up (Negates Y Axis setting)
135 = Shines Diagonally Upward (Reverses Y Axis setting)
180 = Shines Level (Reverses Y Axis setting)
225 = Shines Diagonally Downward (Reverses Y Axis setting)
270 = Shines Down (Negates Y Axis setting)
The light source rotates on the Y and X axis (see Picture). So:
Y (controls 'left to right'):
------------------------
0 = Shines North
90 = Shines West
180 = Shines South
270 = Shines East
45 = Shines North West
135 = Shines South West
225 = Shines South East
315 = Shines North East
X (Controls 'up and down'):
-----------------------
Important angles
0 = Shines Level
45 = Shines Diagonally Upward
315 = Shines Diagonally Downward
Other Angles (I recommend you ignore these
)90 = Shines Straight Up (Negates Y Axis setting)
135 = Shines Diagonally Upward (Reverses Y Axis setting)
180 = Shines Level (Reverses Y Axis setting)
225 = Shines Diagonally Downward (Reverses Y Axis setting)
270 = Shines Down (Negates Y Axis setting)