Caleb Mingle and myself decided to build this last night, it took 3 hours to make the main site and iPhone version. The general idea is to allow people to put in a Twitter username and the site will tell you when it's best to send any @replies or direct messages to that user.
It's worked out by finding the day and time they tweet most assuming this is when they are most active (obviously).
This can be handy if you enjoy speaking to people with a rather large follower base as you know when you're most likely to get a reply or even if you'll get a read at all.
www.tweetoclock.com