Among "tools" I would include Linux, and the innumerable lesser tools that it allows you to incorporate.
For Windows I use many free tools, e.g.:
Audacity - digital audio editor
Kompozer - html editor
Notepad++
Ghostscript - interpreter for PostScript and PDF
InfraRecorder - one of several good optical disc burners
VideoLAN VLC media player
SUPER - Simplified Universal Player Encoder & Renderer. A GUI to FFmpeg, MEncoder, MPlayer, x264, musepack, monkey's audio, true audio, wavpack, ffmpeg2theora and the theora/vorbis RealProducer plugIn.
GIMP - GNU Image Manipulation Program
not open source, but free: Defraggler and CCleaner, Spybot S&D