I don't like your internet site, so I'll saturate your machine with ridiculous amounts of external requests and break your capability to respond to legitimate traffic; a typical DoS attack. Traditionally DoS attack capabilities are a packaged functionality of the notorious "bots". These bots are synchronized-and-controlled using botnets and used to launch DDoS attacks against victim site(s) rendering them incapable to respond to legitimate traffic; a massive thorn for IT.
Nowadays it seems you don't need a complex botnet setup to launch your own DoS attack. This malware sample Troj/Pointu-A is an example of a DoS attack tool which a corrupt minded individual will use against some legitimate site.
This sample has a set of capabilities which theoretically promises to pack a mean punch. The program when run opens a user specified number of threads (in the figure it is set to 5) and each of those threads handle multiple connection attempts. It also comes with this most interesting message window when I clicked on "About".
The author of this program included his ICQ number in the message window and the window title translates "Don't believe him" ... and I wouldn't.