Here you will find two great programs for the Psion Series 5: Flip and Race Form Cracker.
Flip was simply a first attempt at programming on the Psion and is a simple, but fun puzzle game. It's free, so you can download it and do what you like with it. The source code is available too. It's not the neatest code around, but it works!
Race Form Cracker is a powerful application for horse racing fans. Using just the data printed in daily newspapers, Race Form Cracker will identify the horse with the best chance of winning in any race.
Psion Links:
3-Lib,JB Soft,McAleely.
Also here are some programming articles for anyone learning OPL on the Psion Series 5. This isn't a tutorial for beginners, but a collection of handy routines with explanations. You can learn from these routines and adapt them to your own programs.