I use Windows most of my time. Here is a list of open-source free software I often use and recommend to others.
Free software mostly has a deep learning curve, but you get the same functionality (often more) than commercial software. Invest a little time and save money!
F.lux makes the color of your computer’s display adapt to the time of day, warm at night and like sunlight during the day. It helps you sleep better after using your laptop at night. Available for Windows, Mac and Linux.
Firefox: open-source web browser made by people
Thunderbird: free email software (includes good Addons for calendar and tasks)
LibreOffice has all the features that Microsoft Office offers, but is not as user-friendly as the later.
FileZilla is the best software for managing FTP.
Notepad++ is the great notepad application (only for Windows). A haven for programmers!
For the cloud, I recommend Mega.co.nz, a fully encrypted service offering 50GB for free. You cannot share files with others, so use it for personal online backup. It has a desktop application for syncing files similar to Dropbox.
Create Synchronicity can be used to sync files to an external hard drive (or local drive) for backup.
7zip is a lightweight compression program. Can read/write most of the zip formats. Just right-click a compressed file and see options.
FFmpeg is a very powerful audio/video editing tool that can convert, resize, reformat, merge, play, record and stream audio and video files. Hard to use, but very powerful.
ImageMagick contains tools for editing pictures and bitmaps. I used it to batch process scanned pages of books.
RSSOwl is a free and powerful news feed reader. RSSOwl lets you gather, organize and search news in a convenient, easy to use interface with endless flexibility.
Scribus is an open-source desktop publishing software and a free alternative to Microsoft Publisher and Adobe InDesign.
Calibre is a great software for organizing your ebooks. It has features to organize, convert and manage ebooks in various formats.