I use macOS... moving to free software...

First steps

Get a feel for how free software can replace proprietary software that you're using...

Audio and video

• Audacity is a free software audio editor featuring a wide variety of filters, plugins, and codecs.
• Handbrake is a free software video format converter, allowing you to transcode your existing video library to formats like WebM or Ogg Theora.
• VLC is a drop-in and ethical replacement for iTunes, and supports an enormous variety of different file formats and encodings.

Chat clients

• Adium is an all-in-one free software instant messaging client that replaces iChat, supporting the protocols used by Windows Live, Jabber, ICQ, IRC, XMPP, and more.
• Element is a decentralized chat client providing an easy and ethical replacement for apps like Discord.

Email

• Thunderbird is a free software mail reader that replaces Apple Mail and other proprietary clients.

File-sharing

• Cyberduck is a popular FTP and SFTP client with compatibility for other server types as well.
• Transmission and qBitTorrent are two clients for the popular BitTorrent file-sharing protocol, and replace spyware like uTorrent.

Fun and games

• 0 A.D. is a real-time strategy game blending historical accuracy with advanced 3D graphics, and is fully free software.

• Dungeon Crawl Stone Soup is a popular "roguelike" role-playing game, sending players into an always different, randomly generated dungeon for treasure-hunting and exploration.

• Stellarium is a planetarium browser that brings the night sky to your computer screen, allowing you to learn about the final frontier in complete freedom.

Graphics

• Blender is a fully free 3D graphics editor, supplanting nonfree programs like Maya.
• The GIMP is a full-featured and ethical replacement for Adobe Photoshop.
• Inkscape is a vector graphics editor that offers an ethical replacement for Adobe Illustrator.
• Krita replaces Microsoft Paint and other raster graphics editors.

In the office

• Calibre is a free software tool for managing your ebook library, and is additionally able to convert to and from many different ebook formats.
• LibreOffice replaces all of the tools comprising the Microsoft Office suite, allowing you to create documents, spreadsheets, presentations, and more.
• Scribus is an all-in-one desktop publishing application, and is fully free software.

System utilities

• GrandPerspective is a free tool for visualizing the space currently used or free for use on a filesystem.

More free software...