I came across a ECM archive that I need to extract.
The Unarchiver can't handle this format, and I'm not sure about other utilities.
How do I extract ECM in Mac OS X Lion?
You can use the ECM tools created by Neill Corlett, the creator of ECM. Just run "unecm" on the file.
If that seems intimidating, there is a youtube walkthrough here that uses a GUI and contains links to the tools used.