I noticed that a lot of people who are using SteamOS are interested in installing additional applications on it, which is why I decided to make a guide. If you don't understand something or it doesn't work for you, please leave a comment.

Note: This guide is for SteamOS 2.0 and newer. If you're on an older version of SteamOS, replace all instances of the name jessie into wheezy and it should still work.

First you are going to need a complete SteamOS installation, otherwise this guide is probably not going to be of much use.

1. In big picture mode go to Settings -> Interface and enable the desktop.
2. go into the desktop by going to Exit -> Switch to desktop from the start screen of big picture mode.

Now you should see your desktop. Make sure you have at least a keyboard plugged in!

3. Drag your mouse to the top-left corner of the screen, type terminal and hit enter to open a terminal window.

If you don't trust me you can check what the commands in this guide do with the man command, If you want to know what apt-get does for instance, like this:

4. Now we're going to change the password of the desktop user so we can use sudo. Run the following command and pick a password:
5. Open the sources list for editing with the following command
6. Go to the bottom of the file and paste in the following lines(with right-click or ctrl+shift+v):
You can also use a different mirror if you want to, this one is in the Netherlands and very fast for me. More mirrors can be found here: http://www.debian.org/mirror/list

7. Close Nano with ctrl+x and pick y to save the file
8. Let's make sure no SteamOS packages get replaced by the updater by creating a preferences file with the following command:
8. Copy the this to the file and save it like before:
9. Now run the following command to index the new repo:
10. You can now install applications from the debian repo, this can be done in 2 ways. You can either use the apt-get command, if you wanted to install xbmc for instance you'd do that like this:
Then for finding applications you could either search for them here[www.debian.org] or use the apt-cache command like this:
You can also use a gui a tool to do this, like add/remove packages which is installed by default, but you won't be able to find things in the Jessie repo unless you set the pin priority to 110. This can bite you later, but it's unlikely. Pin priority -10 is safer.

Edit1: Fixed the apt-pinning, thanks Hitsuji.
Edit2: Added -t option to installing applications.
Edit3: Fixed a mistake I made on gui tools.
Edit4: Removed some outdated info and fixed some spelling errors.
Edit5: Changed pin priority to 110, since a lot of people seem to have problems with it being set at -10.
Edit6: Updated guide to SteamOS Brewmaster, since that is what Steam Machines seem to ship with.
Edit7: Added warning for SteamOS Alchemist users.
Edit8: Removed some outdated info and some grammar fixes
Edit9: Changed SteamOS repo pin to be more specific