Flexible open-source content management and blogging software with an active user and developer community and a wide variety of plug-ins or extensions available.
Very extensible CMS platform based on PHP and used by many NGOs - "Community Plumbing"
Firefox is the popular open source web browser from Mozilla
Free/cheap Internet telephony and handy IM tool.
Web based social bookmarking lets you share online finds and discover new ones. Ground zero for tagging.
Web based photo sharing service
Apple's desktop operating system.