CLR baby. Just make sure you have the windows open.
CLR comes in a grey jug and can be found at Target, Walmart, sometimes grocery stores (large ones), hardware stores, etc.
This here is CLR: http://www.jelmar.com/CLRproducts.asp
I’d try vinegar first, because it’s cheapest. I’ve always had good luck with it and water spots. Get a sponge with a scrubber back on it and a gallon of plain white vinegar. Dump the vinegar into a bucket (don’t dilute it) and dip the sponge right in it. Srub the shower doors thoroughly, then rinse them with clear water and dry them off (with a squeegee, if you have one).
The only drawback about using vinegar is that your whole house smells like a giant salad…
Now if I could only find out how to clean the stains off of a fiberglass shower pan.
Any suggestions?
I’d try the Mr. Clean Magic Eraser there, too.