For me personally, there's no dilemma. I was landed a pretty good, high-paying job after showing my pretty sophisticated pet projects I made for the fun of it. In my free time, I keep making pet projects out of "curiosity, enlightenment, and purity", and if some have potential, I showcase them to the employer and they're integrated into the product, or as a dev tool. Builds a good resume, too. Maybe I'm lucky that my employer is open to new ideas/projects. So, both the angel and the devil are satisfied :)
Do you keep ownership over those tools when they're integrated into your employer's product? What are the terms on your employment contract? This can get muddy.