Eurobabs, the Niederlassungerlaubnis is the new version of the unbefristete aufen. (UA). THey changed the name of it a few years ago. I know because I got my UA just before the official changeover. I don't think any major conditions were changed with the name change. My husband got his less than a year ago and it has the new name (NE) and a photograph (my old version UA has no picture). I know that with either version you can work as a freelancer or employed without any further fuss or application for a work permit.
Someone asked about the time limit. You must live
and work (i.e. pay into the sozialversicherung/steuer system) in Germany for five years. Any gaps where you were not paying into the system, whether it's due to unemployment or freelancing will not count towards your five years. I had a student visa for a while and that didn't count, either. They added up all the time that I was employed (angestellt) and paying into the system. When it totalled five years I was given a permit. I had actually been living in Germany for nearly 7 years at the time that they gave it to me! I was so pissed off!
Since we're on the subject of annoying visa shit, my son wasn't awarded German citizenship although I had been living here for 8 years when he was born (in a Munich hospital). The rule on that one is that at least one parent has to have had the NE for three years already at the time of birth. The fact that both of us had been living here and been good tax-paying citizens for years meant absolutely nothing. When the German government says they support integration they are LIARS

Then they went on to give our son my husband's nationality against our wishes. Because only the father's nationality counts according to German law (chauvinist bastards!

) unless you go to an Amt and specify otherwise, and then they will only give the mother's nationality as a
second nationality. Has it ever occured to them that some women lie about who fathered their baby. (not saying that I did of course).