The LLVM Compiler System is being used for all sorts of interesting things these days. It basically provides an extremely modular and easy to use set of open source (BSD-licensed) compiler libraries that can be used to build various applications from. Despite this, many people don't really understand it, and are scared away by the breadth of the project or by compilers in general. The Kaleidoscope tutorial starts out from scratch and slowly builds up a simple language to show how LLVM can help out with this. We end up with a JIT compiler for a fairly interesting little language with less than 700 lines of code. Of course, this is just the tip of the iceberg, once you start with LLVM, there are all sorts of things you can do.