While you and I may consider ourselves to be mod_rewrite experts, the truth is that sometimes we need a sanity check before hitting
:wq in vim during the DevOps you know better than to be performing on that production server!
This handy online tester allows you to enter your requesting URL, your .htaccess rules, and check to see how the rules are applied and your rewritten output URL will be.
In particular this saved me from doing cringe-worthy DevOps on a production server where I needed to perform a 301 Redirect when a particular querystring was matched. Not something I wanted to trial-and-error or mess around with lower environments over.