Thomas Linden tom@co.daemon.de [...]
The question now is, who owns the copyright on the software? From my point of view it is me, but I'm unsure. And, would it be legal to publish the software as opensource (say GPLed)?
If the writing of the software was done *for* that company, I think they may hold (some of?) the copyright. Whether it's legal to publish as free software (I'm not sure which "opensource" you mean) depends on what copyright or licence you have. If you were freelance or they broke whatever agreement you had, that may change things, but I'm no lawyer so I don't know.
URL:http://www.iusmentis.com/copyright/crashcourse/ownership/ Arnoud Engelfriet's site may explain this better.
I'm located in germany, so german law applies.
You may find asking a German-centric list gets more informed answers. Try http://lists.gnu.org/mailman/listinfo/fsfe-de
Best wishes with that,