Aptitude stuff
aptitude search mysql # Look for something
dpkg -S `which tsclient` # What package does it belong to?
dpkg -L aria2c # What does this package provide?
dpkg -i *.deb # Install a deb file
dpkg -s nodejs # Show info
dpkg --get-selections # list installed packages
Apt archives path
/var/cache/apt/archives
List services
service --status-all