I'm working on a software problem at work that is fairly generic, but I can't find a library I like to solve it, so I'm considering writing one myself (at least a bare-bones version). I'll be writing some if not all of the 1.0 version at work, since I need it for the project. If turns out well I might want to bring the work home and polish it up just for fun, and maybe release it as an open-source project. However, I'm concerned that if I wrote the 1.0 version at work I may not be allowed to do this from a legal sense. Obviously I could ask my boss (who probably won't care), but I'm curious how other programmers have dealt with this issue and where the law stands here. My one sentence question is, When is it okay (legally/ethically) to open-source a software tool originally written by you for work at work? What if you have expanded the original source significantly during off-hours?

Follow-up: Suppose I write the whole thing at home on my time then simply use it at work, does that change things drastically?

Follow-up 2: Note that I'm not trying to rip off my employer (I understand that they're paying me to build products that they own)--I'm just wondering if there's a fair way of doing this for all involved... It would be nice if some nonprofit down the road could use my code and save them some time. Also, there's another issue at stake. If I write the library for a very simple, generic thing (like HTML tables in Javascript), does that mean I can never again do so on my own time without putting myself at legal risk (even if it was a whole new fresh rewrite or a segment of a larger project). Am I surrendering my right to write code for this sort of project for the rest of my life (without this company's permission), since the code at work might still be somewhere in my brain influencing me? This seems related to software patents, as a side-note.