Aptitude-based distributions (Ubuntu, Debian, etc): dpkg -l
RPM-based distributions (Fedora, RHEL, etc): rpm -qa
pkg*-based distributions (OpenBSD, FreeBSD, etc): pkg_info
Portage-based distributions (Gentoo, etc): equery list or eix -I
pacman-based distributions (Arch Linux, etc): pacman -Q
Cygwin: cygcheck --check-setup --dump-only *
Slackware: slapt-get --installed