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.
Free/cheap Internet telephony and handy IM tool.
Web-based project management tool
Mozilla's Open Source email client
Twitter is a global community of friends and strangers answering one simple question: What are you doing? Answer on your phone, IM, or on the web.
Email marketing/newsletter tool that tracks open rates, click-throughs, etc, segmentation support, and an "infinite" number of lists.