.htaccess tester
htaccess.madewithlove.be
htaccess.madewithlove.be
nginx itself reports clear and easy to understand errors on it's configuration files. If you simply want to test the config (without actually starting/restarting nginx), just pass it the '-t' flag; similar to apache's 'configtest' option.
Edit: Also, just noticed that RewriteCond %{SERVER_PORT} 80 does not work unless you explicitly add :80 to the URL.