KLEE is pretty cool. To compensate for the atrocious build instructions there's a docker image which contains KLEE built and ready to use (https://registry.hub.docker.com/u/kleeweb/klee/).

There's also a web interface to just play around with KLEE without having to download and install anything that a few other people and I worked on available at http://klee.doc.ic.ac.uk:55080/, which we open-sourced https://github.com/klee-web/klee-web.