Introduction: Use a Worn Cassette Cog As a Singlespeed Cog
Singlespeed cogs are stamped, cheap, with deep valleys to avoid throwing chains. Cassette cogs are machined, expensive when new, have low profile teeth, and interesting ramps to aid shifting. Really nice singlespeed cogs are expensive, machined, with tall teeth.
USED cassette cogs can be free, but their teeth may be worn down, which skip, because the worn 'pulling' tooth edge is angled. Not good, right? This is called "shark toothing" by some people, and no, it's not good.
BUT! You can flip the cog around and mount it backwards, to use the unworn 'back' face of the tooth as the new 'pulling' face.
Step 2: File the Keyed Notch to Flip the Cog
BUT NOT YET! Cassette cogs are made with one inside notch 'keyed' to the cassette. The inside part of the cog that slides onto the cassette carrier body has one gap partially filled in so that you can only put the cog onto the cassette in one place. This is so all the ramps for shifting line up properly between cogs, but it also means you can't just flip the cog over and it on backwards.
First you need to file off the little nub, or otherwise alter the 'keyed' notch so it looks and works like all the other notches. All you need is a decently sharp file and about 40 seconds. It doesn't have to be perfect, because the whole enterprise doesn't depend on this one crappy notch.
That's it - now you have another cog to mess around with!