Twitter Auto-Responder?

I'm not sure if something like this exists, but if it does, I'd really like to know about it. I was searching for a Twitter application with a feature to automatically respond to anybody who sent a direct message to your account. I know there's software that's capable of mass-messaging everyone, but that doesn't seem very targeted at all. Also, due to paranoia, I feel as if doing something like that would be much riskier. By responding to messages other people have sent, you'd be targeting an audience who is engaged with the direct-messaging system, and more importantly, they've actually recognized your account.

Anyhow, does such an application exist? Preferably free, because I don't has money yet.

try tweetlater works fine for me

bad idea bud
you are going to respond to spammy bots the same like you
or you think normal people will post you DMs???
Believe me all DMs in your inbox are autoposted so nobody ever care for reply