They paid you for it, didn't they, and to some degree or another directed you in the work. Therefore, in the eyes of copyright law it's “a work made for hire.” Now, I.N.A.L. but, that does mean that to some extent or another you're going to have to explicitly negotiate.

But first, you need to very-seriously consider the actual commercial viability of the work, and whether-or-not you really want to be in the software business. (Let me preface this by saying that I have been in that business, concerning software that I wrote, for over 10 years now.) It's very tough work and not always as profitable as you think it should be.

Sometimes the very best benefits are indirect ones. Put your name on it, negotiate with your employer to permit disclosure of the work, and let it be known that you'll support it. Reputation is a good thing. The “real value” may come from a block of software that is incontestable proof that you know how to devise and maintain something that has real business value to someone; something that “makes things easy and saves hours of work.”

It is very important that you explicitly document that you have dealt forthrightly with your employer, that you have done so on particular established dates, and that you have done so in writing with signed copies of your letters-and-discussions kept by both parties. You don't need to be, nor to get, a lawyer:   you just need to “get it in writing.” May it never come to that but ... the one thing you see time and time again on those court programs is the judge exasperatedly saying, “Look, you say that you said this, and you say that you said that, and I've heard three different dates from the both of you by now, and t'hell with it.”