SnapModbus
32/64 bit multi-platform Modbus communication library
...It supports all the functions described in the documents:
* MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1b3
* MODBUS MESSAGING ON TCP/IP IMPLEMENTATION GUIDE V1.0b
* MODBUS over Serial Line - Specification and Implementation Guide V1.02
through the standard transport protocols and the most widespread unofficial protocols (UDP, RTU Over TCP, RTU Over UDP)
It’s released under lglp v3, so it’s free also for commercial use.