Ubuntu has published its own source, Ubuntu Font Family. In Ubuntu 10.10 it is already available through the official repositories, so it is very easy to install. But, How to install it on older versions of Ubuntu or on other distros?
Ubuntu 10.10 :
sudo aptitude install ttf-ubuntu-font-family
Rest of the world :
wget https://launchpadlibrarian.net/56648584/ubuntu-font-family-sources_0.68%2Bufl.orig.tar.gz
Next, extract the file and double-click on the different TTF files. When they open (see screenshot above), click the Install button.
The Ubuntu Font Family is free to use and is licensed under the Ubuntu Font License (1.0).
Source: font.ubuntu.com