If you want to get the STABLE RELEASE of the ubuntu second life viewer, eg the same as the lindens official release then add the following lines to your /etc/apt/sources.list (you will need to do this as root e.g. sudo vim /etc/apt/sources.list):
# Second Life (Open Metaverse viewer)
deb http://ppa.launchpad.net/openmetaverse/ubuntu hardy main
deb-src http://ppa.launchpad.net/openmetaverse/ubuntu hardy main
Note: If you are not running hardy heron but instead are running a later version of ubuntu then you can change the "hardy" bit to e.g. "intrepid" or "jaunty" and it should work just fine.
Update lists:
sudo apt-get update
Install Second Life:
sudo apt-get install omvviewer
You should then find in the Applications menu, under games is the "Open Metaverse viewer" aka second life!
Ubuntu 8.04 Hardy Heron Second Life Open Metaverse viewer