Note that I have no agreement with my employer, a non-profit, that they own any part of my code.

Agreement or not, if you developed the code on your employer's time you'll find that in many jurisdictions, that employer has some vested interest (aka ownership) in your code.

Am I correct in inferring from comments re distributing to co-workers that you did at least some of the work on the non-profit's nickle?

On the other hand, if you did it on your own time, and merely gifted fellow employees with something they like but that has not become, strictly speaking, an integral part of the business procedures of the non-profit, sell it, if you can. I'm far from a doctinaire advocate of the position that "software wants to be free." If you can sell it, and doing so passes your "smell test," go for it.

But (!) that's going to be a real challenge and not likely to be putting you on easy street, unless you've written the next must-have, killer ap. However, good solid software, aimed at a market you can penetrate, providing unique capabilities or ease of use, may provide you with some pocket change (or better) via dragonchild's suggestions above.