If you’re looking for a free Programmable Logic Controller, look no further than the OpenPLC project. It is compliant with the IEC 61131-3 standard and can work with many hardware platforms; this includes both Arduino and Raspberry Pi, along with commercial PLC hardware. The project has two main components: the editor, and runtime. It supports: Ladder Logic (LD), Function Block Diagram (FBD), Instruction List (IL), Structured Text (ST), and Sequential Function Chart (SFC).
The source code is available on GitHub from Thiago Alves. Or you can visit the OpenPLC website.