A Fink MacOS X port is available here:

An interesting doc covering ladder and grafcet (sequential) for ClassicLadder is available here:

The EMC "Enhanced Machine Controller" project use ClassicLadder in an older and modified version, there is a doc than can be interesting even if some parts are specific to EMC:
Main site for LinuxCNC here (ISO LiveCD available) :

A site about automation with many, many informations and educative topics on it:
(and now a dedicated ClassicLadder page here ! :

Ladder logiq compiler that generates native PIC and AVR code for these microcontrollers:

Olinuxino PLC project (ClassicLadder running on a OLinuXino (Olimex) card):