Open Source

Open source is a sub-category of "completely free": everything here is free to use, and in addition the source code is visible and available.

For an engineer that matters for three practical reasons. You can check what the calculation actually does rather than trust a black box. You can adapt the tool when your problem falls outside the standard. And most importantly — if the project closes or is acquired, the code remains and your working files do not become hostages.

The common licences here are GPL, LGPL and BSD. All permit commercial use; the differences between them concern anyone distributing a modified version of the software, not someone simply using it.