Ubuntu has quickly become the number one Linux distro for the desktop. Not only is it free, but it has also made Linux easier to use than ever. Now, Wubi enables Windows users to install Ubuntu just like any other application, so you no longer have to mess around with partitions, burning CDs, etc.
Be advised that Wubi is just an installer, i.e. it still has to download Ubuntu, ca 700 MB in size, from the web. The software is still beta.
Interesting info from the Wubi FAQ: