If you are looking to learn about software, there is a treasure trove of source code and resources available on the Internet.
One of the more interesting resources is from David Pilling. He is something of a RISC OS legend (being behind OvationPro and many critical tools for RISC OS). His website lists his software and also includes several free utilities.
There is also an interesting section for 'retired' software here. Rather than just let the packages disappear, the source code is now available for all the ArcFax, Panorama, SparkFS and many other former commercial applications. Enjoy digging....
[Updated 18 January] Thanks to Sprow for pointing out SparkFS does not include source code
David Pilling website