Distributed denial of service isn't just something you can download, if you want actually want to crash someone offline or take down an SA-MP server. I'd recommend you visit the hack forums and look for a booter/stresser, they do cost money - the good ones. Legion Stresser, or Illuminati Stresser.
http://hackforums.net/forumdisplay.php?fid=267
If you're looking for something free, don't search for one because it simply doesn't exist. It takes money to ddos, these Legion and Illuminati stressers couldn't of started their business without money - they also need purchases to maintain their business. So, yeah.
Maybe if you had a bunch of computers and used a free ddos program on each of them, like... I'm talking a buttload then maybe you could hit something offline.