Further reading and application notes
A very good and compact primer on STM8S-Programming. Does not rely on any external libraries.
A series of articles on bare metal programming of the STM8S CPUs. By far the best introduction I found so far. It would have helped me a lot if it would have around at the time when I started this project.
Quick introduction to the Arduino.mk makefile on hackaday.com
It contains the SPL examples from ST, one of the most useful resources on the STM8.
Using the ADC by Mark Stevens
AN3139: Migration guideline within the STM8L familiy
Similar or related projects
None of these projects are related to or part of Sduino. They are written independently, but with a similar goal in mind: To simplify STM8 programming for the beginner.
STM8Sduino: A minimalistic approach to adopt parts of the Arduino-API to the STM8. Based on IAR and COSMIC compiler (Windows only). Port to SDCC shouldn't be too hard but is not done yet. Low overhead, but basic functions only. No support for porting Arduino libraries.
Other interesting STM8-related projects
gdb support for the STM8 STM8 toolchain with GDB debugger and SDCC. Use the hardware debugging feature of the STM8 with ST-Link and gdb.