A while back I remember reading about this site that allowed you to game Digg and StumbleUpon called Subvert and Profit. It has been covered on Valleywag, Techcrunch, Search Engine Journal and even Wired just to name a few big news sources.
Pretty much all of them agreed that the service was doomed to fail from the start. Most said they would get so much legal hassle from StumbleUpon and Digg that it would never make it. Well yesterday they quietly changed their pricing structure a bit. It now costs $2.00 per Stumble or Digg and also the users are now getting paid $1 per. That looks like a 100% increase on both sides.
I recently shot a email to the owners to ask where they were based out of and how many users they had. They replied that they were U.S. based and had over 10,000 users now.
I have talked to some friends who have used their services and they say that it works very well for stumbleupon but so-so for digg.