I am wiring my basement with 3 circuits for about 24 receptacles. We used to have electric baseboard heat and the 20 Amp Double pole breakers from the heaters are still there but disconnected. Can I use each double pole breaker for two separate circuits? Can I use one leg of another double pole breaker for a circuit? Am I being dangerous as well as a cheapskate?

Hi,
Yes you can use both sides of the unused breaker each for different circuits, and the 20 Amp rating calls for 12 AWG wire to be correct.
I am unsure about the other; do you mean to say it is unused? If so then I would say yes. If not I would consider another alternative. One might be to change a single breaker to a double, if you have no room to add another breaker.
The important consideration is that the totals for all circuits used do not exceed the rated amperage of your box/service.
Peace,
Clarke

A typical single pole circuit breaker, new, only costs about $4-$6 each.

If you need three, that would be the most $18.00.

Splurge and buy new circuit breakers.