For ladderwork see
http://www.microshadow.com/ladderwork.php
Idea is to compile ladder diagram into HEX file, programmed into MCU.

For my PLC - once a friend of my asked me to replace mold machine controller. I had board with MCU,LCD and 6-button keyboard, some I/O pins.
Since i didnt know how exactly machine works and machine was in other town , i decided to prepare something like PLC. As result my device has some interpreter , with limited set of commands,
variables are - input/outputs and timers. Programm is stored into serial eeprom, paged at about 100 bytes/page(32steps x 3bytes each step + some bytes - header and check sum).
Program is entered trough LCD/keyboard dialog.
Each page is loaded into XDATA and executed step by step. There was additional page ,loaded in other XDATA place, which contains steps executed by timerticks. Mold machine works reliable with new controller. There is one needed feature - potentiometer scanning command for time_variable tuning.
For me this task was funny.