Ubuntu

Which package contains file

$ apt-file search filename

List contents of package

$ apt-file list packagename

Update apt-file database

# apt-file update

show installed packages in cronological order

cat /var/log/dpkg.log* | grep "\ install\ " | sort | less

Installed packages.

dpkg --get-selections
apt-get install openssh-client openssh-server

List packages on one machine and install on another.

dpkg --get-selections > installed-software
dpkg --set-selections < installed-software
dselect

Upgrade.

sudo apt-get -y update
sudo apt-get -y upgrade

Which package provides.

http://packages.ubuntu.com/

List available linux packages.

apt-cache pkgnames

install local deb packages

sudo dpkg -i 1.deb 2.deb 

install from experimental

# add next row to /etc/apt/sources.list and do an apt-get update
deb http://ftp.debian.org/debian/ experimental main contrib non-free
# Then install latest version of ekiga.
apt-get -t experimental install ekiga

add gpg key

gpg --keyserver subkeys.pgp.net --recv-keys AED4B06F473041FA
gpg -a --export AED4B06F473041FA | sudo apt-key add -