I have several wonderful programs I use every day that were developed this way. Some fool asked me to do some silly thing. I took the opportunity to make a more general purpose tool, and viola, I have a product. Of course you have to be careful of invention agreements and non-dosclosures. When they gave me the invention agreement (short version, "we own everything, even if you make it on your own time"), I took their cover page and proceeded to write my own, which I signed. It was still fair. Anything they paid me to make was theirs. Anything else (which included a laundry list of specific tools, libriaries, etc), was mine. One thing about learning about all these formats, you'll be the expert in the field. Once/if you leave, they will be calling you for years with questions and work requests (whether you consider that good or bad has to factor into your decision).

While I was in school, I was contacted by the local branch of a corporation. Between my "internship" job, and getting married, I ended up quitting school (bad move resume wise, but it's worked ok for me). When I finally left their sorry butts, I was shocked to see I'd spent 9 years there. After I quit in September, I ended up making more from them in the rest of the year (as an outside contractor) than my previous annual salary. I did the consulting thing for them and others for a few years, and because of too many details to get into here (mostly involving money and insurance), I ended up back working for them directly. After an initial period where they allowed me to fix all the things my replacements had broken, we proceeded to race as to whether they were going to fire me (I was a troublemaker), or I was going to quit for another 10 years. Finally, they blinked and gave me my severance package a few months before closing the division. I value the experience greatly. As some have stated, it taught me to deal with any insane politics I can ever encounter. Also, when starting my other businesses I'm in now (programming, direct mail, antiques, etc), it has been an invaluable guide to business. All I have to do is ask myself "If my former company were faced with this situation, what would they do?" I then do the exact opposite of that. Works really well.