Proprietary Software [ Edit ]
GNU has great definition of proprietary software:
"Proprietary software, also called nonfree software, means software that doesn't respect users' freedom and community. A proprietary program puts its developer or owner in a position of power over its users. This power is in itself an injustice."
This is a huge problem, because power corrupts and proprietary software usually ends up doing various malicious things (usually in the name of the profit).
More info: Proprietary Software Is Often Malware