Tread carefully. If you’re going to do this, read your current employment agreement carefully especially the parts about Intellectual Property. Every medium-to-large company I have worked at has asserted ownership of everything I build, at work or away from work, using my own equipment, on my own time—it doesn't matter. If your employer can imagne a conflict of interest (for example could what you are working on potentially compete with a current or future business of your employer’s?), you’re screwed. I’ve had to end involvement with open source projects and home hobby projects because of the IP threats in my employment agreements. Feel free to take the risk yourself, personally I don’t feel like fighting a room full of corporate lawyers.

EDIT: jobsandjobs, sorry to have derailed your good Ask HN question but I thought this was an important and often-overlooked pitfall that people should be aware of.