Awlsim is a free Step 7 AWL/STL/PLC simulator.

Depending on the host machine and the Python interpreter used, it achieves
good performance of several thousand to millions of AWL/STL instructions
per second.
German and english AWL/STL mnemonics are supported. 
S7-300 and S7-400 CPUs (with 2 and 4 accus) are supported.

Awlsim has got a hardware interface with existing PROFIBUS-DP (proof-of-concept)
and LinuxCNC plug-in modules.