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.
Full-featured open source office productivity suite, featuring word processor, spreadsheet, presentation, drawing and database packages; free MS Office replacement.
Mozilla's Open Source email client
Gmail is a web-based email with very user-friendly interface and huge storage capacity