LLVM is a great platform for compiler research.  It has a simple and easy to understand <a 
href="docs/LangRef.html">representation</a> and a completely <a href="docs/WritingAnLLVMPass.html">
modular system for adding passes to the compiler</a>.  LLVM also provides extensive assertions
to pinpoint bugs when they occur, an IR verifier, and <a href="docs/CommandGuide/html/bugpoint.html">tools
to automatically identify and narrow down bugs</a> in arbitrary compiler transformations.