twitter viral script ?

Is there any free or paid viral twitter script availiable for html based website , where the script will tweet a pre-written message in excange for download or an information to a visiter
just like a content locker , when the visitor arrives to webpage he need to tweet a specific message that i have set in the webpage then only he will be able to view the content on the website, I have tried with a script and it is not working for me so any input guys

I can't post links yet, but this should work for you - viralmarketingassault dot com

thanks for the link ,It is good and it is free but only down side is that the visitor is redirected to there website before ours chances are there that we will loses the visitor
i wish i had a self hosted script

how about http://www.paywithatweet.com/