valgrind took the world by storm, making it really easy to find memory errors and profile your program's performance. Solaris's DTrace is the killer advance of the last few years. Similarly, Firebug has become a must-have tool for web developers.
# Flash Switcher - (via Zeuslabs) allows you to switch Flash plugin versions on the fly, without reloading Firefox. Supports versions all the way back to Flash 2.
# FlashTracer - With the Flash 9 debug player, you can view your trace information right in