Chalk
This is a mirror/fork of the Chalk calculator by Pierre Chachatelier so I can hack on it.
If you want to download or donate go to these links.
I'm thinking of doing a swift version (Mac and iOS), which may be in a different repo - perhaps DryErase or something similarly groan worthy.
It is released by Pierre under the https://opensource.org/licenses/CECILL-2.1 license, when I make additions, my addtions and thus the whole package with be released under the GPL-3+ license. When this happens, I will delete this message and include the GPL in a LICENSE and/or COPYING file.