Gimp, thats all I can think of for images.
Gmail - I can't live without Gmail.
Google Docs - Online manager/editor of a world of documents. Our whole team runs off Google Docs
Quicksilver(mac) - An app launcher and can MANY other things just using keystrokes
Firefox - enough said
Open Office(.org) - MS office replacement
Notepad++(windows) - My favorite free text/code editor
Netbeans - Mainly a Java IDE but also has others.
Thats all I can think of for right now. I'm not really into image editing software. Although I do want to download Blender sometime soon, and start learning that.