Here are some open-source projects I work on at my spare time:
Java Virtual Machine implementation in pure python.
Easy extendable LL parser. This is front end, producing a parsing tree. By applying rules abstract syntax tree gets created. Functionly is very simple functional language to demonstrate framework use case.
BrainF*ck implementation using the CompilerFramework. It contains both interpreter and compiler. BrainF*ck code gets compiled into java byte code (.class) and is executable on any jvm.
Native mobile applications for Git
Used to be a Git Repository hosting service. Was built on top of Amazon Web Services and had very similar approach of charging user only per resources consumed. Unfortunately, no source code to share.