I'm particularly impressed with the visual PEG debugger tool, here: https://ohmjs.org/editor

PEGs are extremely difficult to debug when things go wrong because you get a MASSIVE trace (if you're lucky) to sift through. This tool has rewind/playback controls to dig into what the PEG rules are doing.

Previously, the best I'd seen was cpp-peglib's online parser, here: https://yhirose.github.io/cpp-peglib/

Edit: and now having read through comments here, Chevrotain's PEG tool looks really impressive, although it doesn't use a simple PEG syntax for the input: https://chevrotain.io/playground/