test262 tests are at test262.ecmascript.org
Kangax’s ES5 compatibility table and strict mode tests
Angus Croll’s ES5 strict mode tests
ES5-for-SES repair diagnostics at es-lab.
Search WebKit bugs for "es5" and Good overview of WebKit progress
V8 bugs tagged "ES5", and search V8 bugs or Chromium bugs for “es5”.
The Besen ES5 project on sourceforge
ES5/3, an implementation of most of ES5/strict that runs on stock ES3 browsers.