Ubuntu is a user friendly distribution based on debian.
These images are on an ext3 filesystem and use 1GB of disk space when uncompressed.
root_fs (32-bit ~60MB) MD5 SHA
root_fs (64-bit ~60MB) MD5 SHA
(last updated on the 2011-06-03)
Since Ubuntu is based on debian, keeping it up to date is as simple as running:
apt-get update apt-get upgrade