Free

sourceforge.net/projects/qmodmaster/ 

Nice interface but only works with single register values (no float, ASCII strings etc support)

Paid

simplymodbus.ca/RTUmaster.htm

Poor operation in Windows 7 but does actually work and does support multi register values so actually a very handy app and well priced for what it is at $60.   We use it for development work as once you get the odd thing that isn't quite correct and its ability to crash in Win7 it does the modbus master job great.  The only thing it doesn't support is writing ASCII strings – it reads them fine but to write them you have to enter the individual registers as numeric values.

modbustools.com 

Not a very nice or intuitive interface, doesn't support multi register values such as 32bit, float etc.  Very over priced for what it is.

 

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