Is there a Twitter autoposter based on keywords?

I'm looking for a software that will automatically respond to what people post on Twitter based on keywords in their post.

For example, if they have the phrase "blackhat seo" in the post then I should automatically post a message such as "@theirusername come check out blackhatworld".

I'm looking for a software, preferably free, that I can just leave running all day doing this. Any suggestions or links will be greatly appreciated. Thanks!