I have been migrating to open source softwares to gradually reduce my dependence on commercial softwares.
The list below is by no means exhaustive. Feel free to send me some suggestions.
  • Gimp: Image manipulation program, similar to Adobe Photoshop
  • Inkscape: Vector graphic editor, similar to Adobe Illustrator
  • Foxit reader: PDF reader, just like Adobe reader
  • Scribus: Layout and publishing software, similar to Microsoft Publisher
  • Peazip: File and archive manager, a free alternative to winzip
  • Audacity: Audio editor and recorder
  • yEd: Diagram editor, an alternative to Microsoft visio
  • Art of Illusion: 3D modelling
  • VirtuaWin: multiple virtual desktop
  • VLC: a multimedia player that plays pretty much every audio and video format out there
  • SciDAVis: Data analysis and visualization
  • Sage: mathematics software, a free alternative to maple, mathematica, etc
  • VPhyton: Python programming language plus a 3D graphics module
Here is a good website which compiles open source alternatives to commercial softwares