Congratulations! Having cracked versions out there indicates that your software is both useful and notable, which is a point where many developers would love to arrive at, maybe, one day. As to what to do about it, there are two basic tracks, neither of which is incompatible with the other: 1. The legal route: notifying distribution sites that they're hosting content in violation of copyright law, requesting takedown…
There is a third option which I'm surprised no one has mentioned so far: Ignore them. First, people or companies using the cracked version of your software were never going to buy it. These people do not pay for software, and nothing you do will convince them to pay for it. Secondly, whatever measures you put in place to combat the cracked versions will potentially have a negative impact on your actual paying users -…
It's very bad economics to think of a pirated copy as a lost sale. In some cases the piracy helps drive sales!