OpenComputers uses APIs from a couple of other mods.

To support accepting energy from their respective power systems:
- BuildCraft [1], license: Minecraft Mod Public License, see homepage.
- IndustrialCraft² [2], license none provided with the API, so I assume
  the API is public domain.
- Universal Electricity [3], license: Lesser General Public License [4].
- Thermal Expansion [5], license: Lesser General Public License [4].

To support bundled redstone input and output:
- RedLogic [6], license: MIT License, see thread.
- MineFactory Reloaded [7], license: custom, see thread.

For providing a peripheral and reading their floppy disks:
- ComputerCraft [8], copyright Daniel Ratcliffe.

[1] http://www.mod-buildcraft.com/
[2] http://forum.industrial-craft.net/
[3] http://universalelectricity.com/
[4] http://www.gnu.org/copyleft/lesser.html
[5] http://teamcofh.com/
[6] http://www.minecraftforum.net/topic/1852277-162-redlogic-wip-replacement-for-rp2-wiringlogiccontrollighting/
[7] http://www.minecraftforum.net/topic/2016680-162164-powercrystals-mods-minefactoryreloaded-powercrystalscore-and-netherores-updated/
[8] http://www.computercraft.info/