Here are the things I can't live without:
• TextWrangler (free, not open source)
http://www.barebones.com/products/textwrangler/
This is a really neat text editor. It has so many features, you'll be amazed. It is like the free cousin to BBEdit. Mac only
• Synergy and SynergyKM (open source)
http://synergy2.sourceforge.net/ http://sourceforge.net/projects/synergykm
This allows you to share one keyboard and mouse across several networked computers. It's really handy to only have one keyboard and mouse on my desk with three to four computers. Synergy is available in GUI form for Windows, command line for Unixes and Linux. SynergyKM is a pref pane for Mac OS X that makes it really easy to use.
• VirtualBox (open source)
http://www.virtualbox.org
This is Sun's open source virtual machine. It works really well and has a good Windows seamless mode.
• Songbird (open source)
http://www.getsongbird.com
Face it, iTunes is less than ideal. Songbird is a great alternative, built on Mozilla XUL and it has a ton of plugins. It is pretty stable although it leaks memory if you leave it open for a couple of days. All platforms.