Just as most of Linux's OSes, Ubuntu is free of charge. The new Ubuntu 13.04 is one of the best graphically designed UI. It is designed to do pretty much anything. Ubuntu nowadays is a simple OS for business, personal use and even servers! It's also Open Source, volunteers are also always trying to make this Operating System.
This link gives a free tour of Ubuntu. It's an online app which gives a glimpse of the Ubuntu 13.04 Experience.
Ubuntu Raring Ringtail runs well with as little as 384 Mb RAM. (The GUI installer requires a minimum of 256 Mb RAM, while the alternative text-based installer can run using only 192 Mb RAM.)Netbooks can run Ubuntu Raring Ringtail, which has been optimised for that platform.
The installation takes between 3-4 Gb hard drive space, and 8 - 10 Gb will be needed to run comfortably. (However, at least 25-30 Gb will likely be needed for routine usage.)
If you have an older computer with less memory than this, consider Lubuntu (if 160 Mb RAM or greater), PuppyLinux (if 256 Mb or greater), or DSL (if minimal RAM, limited hard drive space, running from a USBdrive, or running from within another OS). -http://ubuntuguide.org
Here's a Video Review by Nixie Pixel: