<tool> (Or utility program, tool) Any software that performs some specific task that is secondary to the main purpose of using the computer (the latter would be called application programs) but is not essential to the operation of the computer (system software).
The following are some broad categories of utility software, specific types and examples.
disk checker: fsck
file system compression
* Files and directories
list directory: ls, dir
format conversion: atob
antivirus software: avast, Norton Antivirus
firewall: Zone Alarm, Windows firewall
* Editors for general-purpose formats (as opposed to specific formats like a word processing document)
binary editor, hex editor
e-mail notification: biff
chat: Gaim, cu
* Software development
build: make, ant
installation: apt-get, msiexec, patch