GCC Runtime Library Exception
The GPLv3 add-on that keeps programs compiled with GCC from inheriting the compiler's licence, provided they were compiled the right way.
GCC drops pieces of its runtime libraries, such as libgcc and libstdc++, into every binary. Without an exception, every one of those programs would be a derivative work of GPL code. Older GCC releases had simpler exceptions for libgcc alone; when GCC moved to GPLv3 they were replaced in 2009, with GCC 4.4, by a single Runtime Library Exception. SPDX calls it GCC-exception-3.1.
The novelty was the "Eligible Compilation Process" condition. The exception only applies if no GPL-incompatible software took part in the compilation. GCC was about to gain a plugin interface, and the FSF didn't want anyone writing proprietary optimisation passes as plugins while still enjoying the exception. It's a licence that governs how code is built, not just how it's distributed.