When I started working as a salaried employee doing software development, my employment contract included language to the effect that everything I produced using any company resources, or using internal company information, belonged to them. When you're salaried, you don't really have "your own time", and since they're paying you, most companies would say that your time is another "company resource". I'm not sure how well that would hold up in court, but I'd also expect that most companies could grind their employees into the ground if it came to time in a courtroom.
Realistically, they weren't interested in the little hobby game I was writing (my employer produced business software), so it's unlikely that they'll claim copyright on it. Now, if I developed a new plugin for my employer's product, that's a somewhat more danger-fraught proposition.
What you want is probably some time with a lawyer, and to begin negotiations with your employer for an explicit contract stating that you own copyright on the things you're making, but that you're assigning non-exclusive, but unlimited use+distribution+modification rights to your employer (OK, obviously I'm no lawyer, but I'm sure you get the idea).
You may be on good terms with your employer, but they aren't your friend. They're out to make money off of your work. There needs to be a contract outlining who owns what, what they can do with it, etc. Otherwise, you're opening yourself up for bad times.