Another kind of shuriken that I've found to be much sturdier and flies better than the one above. All you need is one square sheet of paper, 8 inches to a side for a decent sized cubicle weapon.

Cut the paper in half vertically so you have two 2:1 rectangles.

|¯¯¯¯| |¯¯¯¯|
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
|____| |____|

Fold each paper in half lengthwise, then crease each piece across in the middle.

|¯¯| |¯¯|
| | | |
| | | |
|..| |..|
| | | |
| | | |
| | | |
|__| |__|

To fit together, these pieces need to be mirror images, so this is an important step. On one piece, fold the top half diagonally right so the previous left edge is against the crease. Then fold the bottom half diagonally left so the previous right edge is against the crease. Reverse the directions for the other piece.

_______
/ |
/________| ______
| | / |
| | -> _____/_______|
| | | /
|___| |_______/
______ ______
/ | | \
_____/_______| |_______\____
| / \ |
|_______/ \_______|

Turn the models over. Where the piece under sticks out the sides, there should be a square. Fold the outermost corner of these squares diagonally so they make a triangle (labled 1). This half of the entire model shall be labled A. (For clarity's sake, I'm only showing one of the models. You should be smart enough to figure out how to do the same on the other.

-> _______
| | \
| | \
|______|.....\ ______
\ | |
\ | |
\ |______| <-
_
/ | \
/ 1 | \
/_____|.....\ ______
\ | /
\ | 1 /
\ |_/

Now fold the other paper this way (piece B). Lay B atop of A upside-down so the slit (line of ":"s) is vertical.

>

|\
_ | \
/ | \ |____\
/ 1 | / : /
/_____| / : /_____
/ : /| /
/____:/ | 1 /
\ |\ |_/
\ |
\|

Fold the 1's horizontally into the slit. This locks the two pieces together. Turn over and you'll see that piece A has it's own slit to tuck the triangles into.

Finished result:

|\
| \
| \
| \__________
| | /
/____|____ /
/ | /
/________ | |
\ |
\ |
\ |
\|

Perfect for warding off Pointy-Haired Bosses, annoying roommates, or those who doubt your Ninjutsu skillz.

My ASCII sucks, so sue me