The Synacor Challenge was presented at OSCON and php[tek] in the past.
In this challenge, your job is to use this architecture spec to create a virtual machine capable of running the included binary. Along the way, you will find codes; submit these to the challenge website to track your progress. Good luck!
I extended a bit the scope of the project: I decided to also write a monitor/debugger for the virtual machine. It seemed very useful to solve the challenge, plus I've never written a QT GUI app in Common Lisp, so this was a perfect excuse.