To see what repository a package was installed via in Ubuntu using apt, use this command:
$ apt-cache policy packagename
Example:
$ apt-cache policy firefox
firefox:
Installed: 3.0.3+build1+nobinonly-0ubuntu0.8.04.1
Candidate: 3.0.3+build1+nobinonly-0ubuntu0.8.04.1
Version table:
*** 3.0.3+build1+nobinonly-0ubuntu0.8.04.1 0
500 http://security.ubuntu.com hardy-security/main Packages
100 /var/lib/dpkg/status
[...]

After installing Ubuntu 8 Hardy Heron at the office, I was having trouble reaching the ‘.local’ DNS entries we have on our internal network. Here’s what I did to resolve the problem:
$ sudo vi /etc/nsswitch.conf
Replace:
hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4
with:
hosts: [...]

Here is how to control which application Firefox uses to handle MMS streams:
Type about:config Firefox address bar.
Right-click on any text in there and select “New” and then “String”.
In the first dialog box, as preference name, type network.protocol-handler.app.mms.
In the second dialog box, as string value, type in vlc (or the path to your favorite media player)

Here’s how to make F-Spot your default application for importing photos from your digital camera in Ubuntu Gutsy 7.10:
System -> Preferences -> Removable Drives and Media. Click the ‘Cameras’ tab. Check the box next to ‘Import digital photographs when connected’. Enter the following for ‘Command:’:
f-spot-import %h
Now when you plug a digital camera [...]