NOTE: These links are somewhat historic. Some may need updating.
Eclipse IDE for C/C++, Java, and other languages
http://www.eclipse.org/
FreePV open source panorama viewer
http://freepv.sourceforge.net/
IrfanView image management software: Free software for photo sorting and some simple editing. Currently only available for Windows.
http://www.irfanview.com/
OpenStreetMap
The open source alternative to Google Maps, Tom Tom and other map and navigation software providers.
http://www.openstreetmap.org/
Processing programming language and environment
https://processing.org/
Sourcery CodeBench Lite Edition. Formerly called CodeSourcery G++ GNU Toolchain for ARM Processors.
http://www.mentor.com/embedded-software/sourcery-tools/sourcery-codebench/editions/lite-edition/
WinAVR GCC for Windows for AVR ATMega processors
http://www.webring.org/l/rd?ring=avr;id=59;url=http%3A%2F%2Fwinavr.sourceforge.net%2F