This is an article for resellers, but it might be interesting to you as well Open Source Xibo and by extension http://xibo.org.uk/legal/
Basically these 3 things are the most important:
- The users know the software is free
- The Copyright notices remain intact (Xibo is Copyright of the Xibo Developers, Daniel Garner, Alex Harrington and James Packer)
- The software (at all times) displays a link to the source code