I love index cards. The lines are a scant 1/4" apart on the ones that I find in the local office supply stores. Following are some of the ways that I use them:
1. check needle placement against the 1/4" seam mark on my sewing machine
2. check 1/4" seam allowance--particularly if I'm doing a technique that requires ironing a 1/4" seam allowance.
3. use as a straight edge to mark the diagonal on a square for making 1/2 square triangles.
4. use to mark the diagonal when joining binding strips--they're thin enough and flexible enough that you can pin the binding pieces together using those flower pins, lay your index card over them and then use whatever marking tool you prefer (I like a sliver of soap for darks and the purple air-disappearing ink for lights). The card doesn't wobble over the pins like a ruler will, and its edge is sturdy enough to make a nice straight line when marking.
5. cut them into smaller pieces to use as labels
Rob