The LLVM Compiler Infrastructure Project

Low Level Virtual Machine (LLVM) is:

A compilation strategy designed to enable effective program optimization across the entire lifetime of a program. LLVM supports effective optimization at compile time, link-time (particularly interprocedural), run-time and offline (i.e., after software is installed), while remaining transparent to developers and maintaining compatibility with existing build scripts.
A virtual instruction set - LLVM is a low-level object code representation that uses simple RISC-like instructions, but provides rich, language-independent, type information and dataflow (SSA) information about operands. This combination enables sophisticated transformations on object code, while remaining light-weight enough to be attached to the executable. This combination is key to allowing link-time, run-time, and offline transformations.
A compiler infrastructure - LLVM is also a collection of source code that implements the language and compilation strategy. The primary components of the LLVM infrastructure are a GCC-based C & C++ front-end, a link-time optimization framework with a growing set of global and interprocedural analyses and transformations, static back-ends for many popular (and some obscure) architectures, a back-end which emits portable C code, and a Just-In-Time compilers for several architectures.
LLVM does not imply things that you would expect from a high-level virtual machine. It does not require garbage collection or run-time code generation (In fact, LLVM makes a great static compiler!). Note that optional LLVM components can be used to build high-level virtual machines and other systems that need these services.

LLVM is a robust system, particularly well suited for developing new mid-level language-independent analyses and optimizations of all sorts, including those that require extensive interprocedural analysis. LLVM is also a great target for front-end development for conventional or research programming languages, including those which require compile-time, link-time, or run-time optimization for effective implementation, proper tail calls or garbage collection. We have an incomplete list of projects which have used LLVM for various purposes, showing that you can get up-and-running quickly with LLVM, giving time to do interesting things, even if you only have a semester in a University course. We also have a list of ideas for projects in LLVM.

If you'd like to learn more about LLVM, take a look at the LLVM Tutorials and the extensive documentation for LLVM. In particular, all of the tools distributed with LLVM are described in the LLVM Command Guide. If you're interested in what source-language features and optimizations we support, please check out the LLVM demo page. If you'd like to browse through the source code, either check out doxygen or download the most recent release. Finally, if you're interested in LLVM, have questions, and can't find any answers, please ask on the LLVM Developer mailing list.