A lot. I don't remember a exact study on popcon top 100, but just going main top 5, let see.
top 1: Gnu sed. License: GPL.
top 2 (if you ignore libs): base-passwd. GPL
top 3: GNU core utilities: Mostly if not exclusively GPL.
top 4: debianutils: SMAIL GENERAL PUBLIC LICENSE
top 5: dpkg: GPL (got bsd parts is distributed as a whole under gpl).
http://popcon.debian.org/main/by_inst if you want to dig through it. A lot of the top is part of base install. Further down we got things like wget, which surprise surprise, is gpl. Could always try to make a debian machine without udev, which is GPL. Honestly, making a debian installation, using debian main archives, without any copyleft'ed software would be a nice project just to see if anything resembling a function operative system would remain. One would have to start by using BSD kernel (which is supported), and go from there to try mimic a bsd operative system.
