From Organiser to Series 5: Programming tools
It's not so far from the truth, that Psion Company has set the standards for many modern portable computers. Their sophisticated, sometimes revolutionary solutions pioneered the development of palmtops. Unfortunately, one of the great features of Psions not followed by today's palmtop market was the OPL. The idea of a built-in programming language wasn't new in those times - all home computers had BASIC interpreter on board - but OPL joined the ease of BASIC with the structural syntax and was well integrated with Psion's built-in facilities. And it was backwards compatible, as long as no system-specific functions were used. This means that you should be able to run your software written for the Organiser on a Symbian smartphone with just a few modifications! 
Apart from standalone programs, you can extend your system with OPL. The Calculator application on Series 3 Psions can perform calculations using built-in and user's own OPL functions. Also macros for Macro5 are written in OPL. On the Series 5 OPL programs can be extended by OPX modules written in C++. However, although OPL is the simplest way to write programs for Psions, it's not the only one.
On the opposite of OPL there are assemblers, allowing you to access directly machine's "guts". Since different models have different processors, it isn't possible to run your program designed for an Organiser on Series 3. Even Series 3 and 5 are incompatible.
Psion Organiser II models have an 8-bit HD6303 processor. You can program it using PCMAC. It's a cross-assembler, i.e. it's run on a PC and the generated machine code must be transferred to the Organiser.
All Series 3 models and the Siena have a 16-bit NEC V30 - which in fact is a low-consumption 8086 clone (although in different models it's clocked at different speeds). This means that you may use an x86 assembler, compile the code on a PC and it will be ready to run on the Psion. There are tons of commercial (TASM, MASM) as well as free- or shareware (A86, New Basic Assembler) assemblers. If you're willing to do the work on your Psion, there's even one that runs natively on your machine - S3A.
Series 5 models are powered by a 32-bit ARM710T. I know of two assemblers for them, both running on the Psion. The first is French Assembleur (unfortunately, the instruction manual is also in French) and the second is called simply ASM. I also read about "GB Assembler Studio" by Nicklas Larsson, but it seems to have disappeared from the Web.
Something else? Sure. Series 3 computers can be programmed in C. You'll find some good resources on Gareth and Jane Saunders' website. They have also some goodies for those who would like to write programs in C++ for their Series 5.
Of course that's not all. There are many more, especially for Psion Series 5. Among them are two Titans: Perl 5.6.1 and Python 2.2.1. (There's also Python 2.1 port, Epocpython). To me, the greatest drawback of these ports is the lack of Psion API (or I haven't found it). So they may be good for prototyping (in a limited manner, as Python 2.2 is an ancient version, eons before 2.7, not to mention 3k) or running some useful scripts (keep in mind, that they'll execute really slow), but nothing more. No real Psion applications written in Python/Perl. What a pity.
For those who want to program in a well-known, multi-platform and powerful language, there's Lua. Not only can you write standalone Lua programs, but also integrate Lua code into your OPL programs, making it possible to write applications utilizing Psion's features, such as GUI. Lua is available from FreEPOC.
You can also run Java programs on your Series 5 machine (Psion runs the Java Virtual Machine, programs need to be compiled on a PC). Among other, less known languages, is Prolog and even... a Brainfuck interpreter
- "Programming Psion Computers" by Leigh Edwards and other free e-books and manuals for both SIBO and EPOC programming
- Programming manuals for Psion Organiser II
- HD6303 and ARM assembly mnemonics are described in the abovementioned packages (Assembler for the Org II) or on their websites (ASM for Psion 5)
- Official home pages, where programming tutorials can be found: Python, Perl, Lua, Java
-  Actually, Organiser's OPL is not a good example, since it uses a slightly different syntax (e.g. no PROC-ENDP statements). But programs developed for the Series 3, even those with GUI, should work on Symbian. ↩