The EPOC / Symbian machines (Series 5, Series 5mx, Revo, Revo Plus, Series 7, NetPad and NetBook) can be programmed using:
There is a separate section on OPX development, and .sis installer development.
The SIBO machines (Siena, Series 3a, Series 3c, Series 3mx, Workabout, and Workabout mx) can be programmed using:
A number of book resources are available, some free, some at a price, for software development on the Psion and Symbian platforms.
There are a number of freely accessible newsgroups (like bulletin boards) dedicated to all aspects of Psion computers, including programming, where you can ask for help, and in turn help others.