Freeware Tools which I find myself using regularily for various purposes:
Audio Editing:
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Audacity
Image Editing:
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IrfanView (Image Viewer)
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GIMP
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Inkscape
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Paint.NET
Video Editing:
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AviSynth (Script-based video editing)
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VirtualDub (Basic AVI editing, AVI screen capture)
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MP4Muxer (MP4 Muxing and Demuxing)
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MeGUI (Encoding)
Media Players:
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MPC-HC
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VLC Media Player
Other:
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JDownloader
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7-Zip File Manager
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HWMonitor
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CPU-Z
Of course, there are other useful tools, but these are just the ones which I use on a somewhat regular basis. I'll add to the list as I think of others.
On that note, I'd also like to make mention of Google and Wolfram Alpha as easy "type something in and get a result" calculators for anything imaginable. These two don't exactly fit the definition of free software however. I'd also mention Notepad++, but it already has been mentioned before.