LibModbus is a simple modbus toolbox for Linux. This library contents master, slave and serial port configuration routines. To know more about libmodbus view documentation.
jamod is a project that provides a Modbus protocol implementation in 100% Java. It can be used to implement Modbus masters and slaves in Serial (ASCII, RTU (Master only), BIN) and IP (TCP, UDP).
The design of this library is fully object oriented, based on abstractions which should support easy understanding, reusability and extensibility.