Dear God, the responses so far are TERRIBLE. Don't recommend you favorite distro with absolutely no justification. What does that add to the thread?
Here's the thing to know: GNU/Linux has many different distributions (alternate versions of the OS) and these each have a philosophy that they use to maintain and build their distro. All GNU/Linux programs can [for the most part] run on all distributions. So you could have all the hacking, cracking, programming tools on ANY distribution of GNU/Linux. However, there are some distros with philosophies of which you could take advantage. For example, Backtrack is a GNU/Linux distribution that is designed to be a penetration testing suite that boots from CD and always has a clean environment. However, you could always download all of those same applications and set up any other GNU/Linux distribution (such as Ubuntu, Arch, Gentoo, Debian, Fedora) to do exactly the same things as Backtrack. However, if you don't want to use GNU/Linux full time, Backtrack is a useful tool to be burned to a CD near you.