One of the great things about Common Lisp is the variety of implementations and the scope of deployment platforms and performance characteristic trade-offs (run-time vs compile-time vs memory size) they encompass. This is also one of the things that complicates library development. Testing on any significant combination of implementations and platforms is not practical for most Free Software library authors.
cl-test-grid is a project that provides automated, distributed testing for libraries available via Quicklisp. You download cl-test-grid on your machine, tell it about which CL implementations you have available, it runs the test suites from many libraries in the current Quicklisp release, and sends the test results to a publicly available report as well as a public bug tracker. The report details test failures by Common Lisp implementations, platform/OS, and library version.
The report is a great resource for library authors and implementation maintainers. If your library is distributed via Quicklisp and isn't tested by cl-test-grid yet, Anton Vodonosov provides some tips for making your test suite cl-test-grid friendly. If you're looking to contribute to Free Lisp software, one of the best ways is to get cl-test-grid and run the tests. See if there are any failures in the test suites, and report the bugs to the library maintainers and help find a fix.