The IVAR system from IKEA offers shelves and pentagonal corner units, but I wanted a little triangular end unit. With a bit of sawing and drilling, I was able to transform each small shelf into two triangular shelves.
As an aside, note that the zigzag long shelves add some extra structural support as they act partly as a truss, in addition to looking interesting.
Step 1: Start with small shelf
IVAR offers a small pine shelf which has reinforcing metal bits for supporting the shelf on little metal pegs.
Step 2: Cut shelf to size
I cut the small shelf along a diagonal to give the triangular shape I wanted. I also chopped out a square notch for the vertical posts that hold the shelf off the ground.
Step 3: Drill peg holes
Around the square I drilled notches for the metal pegs. These notches aren't as strong as the metal-reinforced bits, but the shelf is small enough that it shouldn't have to bear too much weight. Note I botched the peg hole on this particular shelf!
Step 4: Fit the shelf on the pegs
Here you can see the shelf seated on the pegs inserted into the IVAR corner post. Note that since each rectangular shelf comes with four pegs, but you make from this two triangular shelves which also need four pegs, you have to pick up some extra pegs!
Step 5: Finished triangular shelf
Here you can see the finished triangular shelf held up by a corner post and a vertical end unit (constituting the other two vertical posts). The corner post is attached to the back post of the end unit with an X cross-brace (not shown).