Ubuntu is the most powerful Linux sponsored by Canonical. Its an open source project. The main license used is the GNU General Public License. Thus it is available for free download, Moreover you can also request an Installation Disk for Ubuntu 10.04 which gets delivered to the specified address for free(Yes, Even without shipping charges). To download the CD Image file go to http://www.ubuntu.com/getubuntu/download. You might also like to download Ubuntu installer for Windows (Quite convenient if you are on Windows). Downloading is always more encouraged, As it is faster. But, If you are behind a slow Internet connect you can request the CD from https://shipit.ubuntu.com/. The CD gets delivered in some 4-10 weeks. 
You can also download Kubuntu for free from http://www.kubuntu.org/getkubuntu/download and request a disk from https://shipit.kubuntu.org/.