Ubuntu Aptitude

In an RPM-based system, the command

$ rpm -qa

will give you a complete list of packages (RPMs) installed. Or, if you use yum,

$ yum list

will give you a list of all available packages, indicating those that are installed on your system.

There must be an equivalent command in a Debian system, but I haven't found it as yet. However, there is a program called aptitude, which gives you a curses-based (i.e. ASCII graphics) method of scanning through the packaged database. It even gives you hints as to the quality of the software — for example, it tells me that the Intel Fortran package I installed today comes from an untrusted (that is, not in Ubuntu's database) source.

Still looking for something like rpm -qa, though.