There isn't much to say about this, other than to be very grateful it exists. I never saw the original web-based reputation calculator, but I'm told there were sporadic problems with US realms. Grumbles writes and says that he was waiting for the EU calculator to come back online and to support US realms again, when he decided to just write his own. I wish I had that kind of talent.
Visiting wowreputationcalculator.com prompts you to enter your name, server, and whether it's a US or EU server. I presume it then reads your reputation information from the Armory and then processes it in a plain but functional way that shows you how many turn-ins and/or quests you need to reach the next level with most of the key Burning Crusade factions. Let me tell you, I'm bookmarking this puppy right now, even if I'm a little disappointed to suddenly realize how long it's going to take me to get exalted with the Kurenai.
This is a very handy tool for reputation grinding. I've seen some add-ons in the past that will count quest items for you, but this is a really excellent resource. Thanks to Grumbles for making it happen for the US players!