There are numerous sites that you can use to get masses and masses of retweets. The only drawback is that you have participate, but it doesn't really take all that much effort on your part.
They all work on credit systems, whereby you retweet someone's message and you get a number of credits in return. You can then use those credits to have your message retweeted.
You can outsource this process on Amazon's Mechanical Turk for really cheap.
The only question remaining, then, is are these retweets really worth anything?
In my experience, retweets that come from sites like these or from bots will never be worth near as much as a retweet from someone who cares about you or your brand. But still, they can help.
As far as software, I don't think you will have much luck.