LLVM
Modular compiler infrastructure — Chris Lattner UIUC, 2003. Defines LLVM IR (intermediate representation) as a universal target for many frontends and source for many backends.
Pipeline: frontend (Clang for C++, Rustc, Swift, Kotlin/Native, …) → LLVM IR → optimizer passes → backend (x86, ARM, RISC-V, WebAssembly, …). MLIR (2019) is a multi-level IR extension for ML and DSLs.
Powers Clang, Swift, Rust, Julia, MLIR (TensorFlow, PyTorch internals), Mojo, Zig, GHC backend. Apache 2.0 + LLVM Exception. Standard in modern language toolchains.