I downloaded Qgis for Ubuntu 13.04 and I encountered a few issues.
First issue I had was that I was not able to download any plugin-ins. Second issue is that I was having is that I was not able to download and use the sextante plugin after solving my initial problem.
Before starting I would do a clean install of Qgis if you have been having problems
sudo apt-get purge qgis
If you having issues downloading plugins:
I found this tutorial online and it is from a few years ago but the step to remove the directory worked for me.
Change to your home directory in the terminal, if you need help with the terminal see the Ubuntu documentation
sudo rm -Rf .qgis
What these commands will do is delete a read only file that is put on your computer during installation preventing you from being able to download plugins. There is a more detailed explanation in the link.
To download the sextante plug-in:
If you want to use all the parts of the sextante plugin, make sure you have grass installed: http://hub.qgis.org/projects/quantum-gis/wiki/Download and make sure you have SAGA installed. You can get SAGA from the software center or online http://www.saga-gis.org/en/index.html.
If you are getting the error that you are missing psycopg2 you have two options
First try and download it from the software center
Then in the terminal:
sudo apt-get update
Then open up Qgis and see if you are able to get the plugin.
If this does not work for you and when you try and download from the software center it give you an error then here is the process to get around that
In the software center look for the synaptic package manager
What is allows you to is download any updates or repositories needed as a sudo user with a GUI interface
In the search box look for the update: I just typed psy and it came up in the list
Click on it, check mark for installation and then Mark all upgrades
Then in the terminal
sudo apt-get update
Then open Qgis and the plugins should be available.
If you have any questions please feel free to post in the comments below.