Here are some tools I use pretty frequently.
- Stellarium: An open source planetarium software.
- 7Zip: Compression software that supports many compression and archiving algorithms.
- Notepad++ or Sublime: Awesome text editors with each having it's own strengths.
- Audacity: Audio editing software. It can be difficult to use, but it's quite powerful.
- Autoruns: An awesome tool for managing what runs / starts up on your PC.
- WinSCP: A file transfer utility that supports many protocols.
- InfanView: It's a media file viewer that supports formats that many others do not. (like FITS files, etc)
- Inkscape: GIMP was listed, but a very nice vector graphics tool is Inkscape. (there is Blender too, but I really don't do 3D rendering artist stuff)
- VirtualBox: Used to create virtual machines on your Windows desktop.
- HWiNFO: Crazy detailed views of your computer's hardware and status.
- ImgBurn: Burns CDs/DVDs etc. I do not burn near as much as I used too though.
I'm sure there are many more.
Listed in above posts is both LibreOffice and OpenOffice. LibreOffice is a fork of OpenOffice. I would recommend using LibreOffice over OpenOffice though OpenOffice I believe has finally been returned to the true open source. The reason for using LibreOffice is when Oracle bought Sun Microsystems. All the OpenOffice developers left and forked into LibreOffice. So the original developers are with LibreOffice, not OpenOffice.