some of my favorites are GIMP, InkScape, and Sublime Text, Although Sublime text isn't technically free, but it's open source, and is great software.
I know lots of people love Blender (3D Software)
GIMP is good if you're doing photo edits, and Inkscape is good if you're creating vector art. I've tried making a web layout on inkscape before and it just doesn't work for me. I suppose I should give it a try with gimp somtime. If you are doing print work though, neither Inkscape or GIMP are good for you. They both have issues with print work. Sadly, Adobe dominates the design world, and this is one of those reasons.
I also love 7zip and fully agree with MaxCady about how great it is.