Spoiler: I've switched to Linux for my main computer, and here's what I'm currently using.
Fedora creates an innovative, free, and open source platform for hardware, clouds, and containers that enables software developers and community members to build tailored solutions for their users.
Fast and stable operating system.
Plasma is a cross-device work environment by the KDE Community where trust is put on the user's capacity to best define their own workflow and preferences.
As the desktop environment to replace Aqua.
Konsole is a terminal to run a command shell. It provides a command line interface for directly controlling your computer.
Usability-focused file manager with tabbed browsing, dual panes, remote protocol support, archive browsing, integrated renaming, and file type previews.
File manager to replace
Finder. Other good alternatives:
Thunar (if using Xfce) and
PCMan File Manager.
Merkuro is a calendar and contact application that allows you to manage your tasks, events and contacts. Merkuro supports both local calendars as well as a multitude of online calendars: Nextcloud, Google® Calendar, Outlook®, Caldav, and many more.
Calendar app to replace
Apple Calendar.
Darling is a macOS translation layer for Linux. It is designed to run macOS software directly without the need for a hardware emulator, making it fast and efficient.
To use macOS software on Linux. Also:
Wine &
PlayOnLinux - PlayOnMac.
Markdown editor for Mac, Windows, and Linux with real-time preview and distraction-free interface. Open source, free on GitHub, supports various Markdown specs.
A free Markdown editor to replace
Typora (which is now paid).
Stacer is an open source system optimizer and application monitor that helps users to manage entire system with different aspects, its an all in one system utility.
System optimizer and monitoring. Also worth mentioning:
BleachBit
Audio effects tool for PipeWire with limiter, compressor, equalizer, spectrum analyzer and real-time control of input/output, plus profile saves.
Audio utility.
Cronopete is a backup utility for Linux, modeled after Apple's 
Time Machine. It aims to simplify the creation of periodic backups.
To replace
Déjà Dup &
Timeshift
KDE Partition Manager is a utility program to help you manage the disk devices, partitions and file systems on your computer. It allows you to easily create, copy, move, delete, resize without losing data, backup and restore partitions.
Partition manager to replace
Disk Utility. Also worth mentioning:
GParted
Open-source browser with Gecko engine, supporting HTML add-ons and providing privacy features like Enhanced Tracking Protection and third-party cookie blocking.
Main web browser to replace
Google Chrome (Google…) and
Safari. Like I do on macOS, I would use
ungoogled-chromium or
Brave as my second web browser.
Boost your productivity and send better email with Mailspring, a beautiful, fast email client for Mac, Windows, and Linux. Mailspring is free to use and an optional pro version adds read receipts, send later, reminders and more.
Email client to replace
Apple Mail. I've also tried
Thunderbird, but it's too much and too bloated for me.
LucaMail also looks interesting.
Transmission has the features you expect in a BitTorrent client: encryption, a web-based interface, peer exchange, UPnP and NAT-PMP port forwarding, web seed support, watch directories, tracker editing, global and per-torrent speed limits, and more.
This is the BitTorrent client I'm already using on macOS.
qBittorrent also looks pretty good but I've never tried it.
Firewall for Linux providing interactive outbound filtering, blocking ads and malware domains, configurable via GUI, with centralized node management.
Firewall application that monitors and controls outbound internet traffic to replace
Little Snitch.
Cross-platform, free media player supports multimedia files, DVDs, streaming, offers hardware acceleration, 4K, lossless audio, and more.
An open-source multithreaded video transcoder, HandBrake compresses and converts digital media, supporting MP4, MKV formats, and offers hardware-accelerated conversion.
For video conversion/transcoding. It's already what I'm using on macOS.
Ocenaudio is a cross-platform, easy to use, fast and functional audio editor. It is the ideal software for people who need to edit and analyze audio files without complications. ocenaudio also has powerful features that will please more advanced users.
To record audio. It's already what I'm using on my Mac.
Multi-platform document viewer supporting PDF, EPUB, comics and images with annotation tools, sidebar navigation, signature verification, and text-to-speech.
Fast image viewer with support for multiple formats, quick crop, rotate, lossless JPEG editing, drag-and-drop, thumbnail navigation, and slideshow mode.
Web-based vector graphics editor with desktop apps, focusing on real-time collaboration and version control for UI/UX design.
For graphic design. It's already what I'm using on macOS. Also:
Corel Vector &
Akira
Open source vector graphics editor supporting SVG with features like custom brushes, pressure sensitivity, and layer management across multiple platforms.
For vector graphics editing to replace
Affinity Designer. Also:
Vectr
Web-based vector editor for creating SVG files with paths, gradients, symbols, shape tools, Google Fonts, booleans, raster import/export, and grouping.
A nice vector graphics app when I don't want to open Inkscape. It's free on Linux.
Intuitive multi-track video editor with robust hardware-accelerated support; offers chroma key, color grading, and extensive audio/video format compatibility.
Video editor to replace
Final Cut Pro. Also:
Olive Video Editor &
DaVinci Resolve.
Open source non-destructive RAW photo editor and image database with lighttable and darkroom views, real-time editing, HDR and scientific image support.
Photo editor to replace
Adobe Lightroom.
Font Manager is intended to provide a way for average users to easily manage desktop fonts, without having to resort to command line tools or editing configuration files by hand. While designed primarily with the Gnome Desktop Environment in mind, it should work well with other...
The Kate project develops two main products: KatePart, the advanced editor component which is used in numerous KDE applications requiring a text editing component, and Kate, a MDI text editor application.
Code editor to replace
TextMate.
Open-source FTP client supporting FTP, FTPS, and SFTP with features like large file transfers, site management, and cross-platform because of FileZilla Server.
FTP client to replace
Transmit.
I'm using Dolphin, since it can handle FTP/SFTP natively.
I'm not playing games on my Mac, so this would be new.
Steam is a digital distribution, digital rights management, multiplayer and communications platform developed by Valve Corporation. It is used to distribute a wide range of games and related media entirely over the internet, stretching from one-man independent efforts to some of...
For gaming.
Lutris is an open source gaming platform for GNU/Linux. It allows you to gather and manage (install, configure and launch) all your games acquired from any source, in a single interface.
For gaming.
A free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare.
Comments
Yes, good list, here more software https://pkgs.org
Great list, but didn't specify what each one is equivalent to on the Mac. For me, finding Typora.io was a revelation since it is the closest to Bear Notes that I've found so far.
Interesting list, mixing libre software with closed freemium.
Thanks for putting a light on Oni :)
If you prefer KDE integration:
For mail, there's also Kontact which can does what thunderbird does and more (cal, contacts), in a KDE integrated style. For screenshots Spectacle is integrated with KDE, it's got timing options. Although flameshot is great to edit on the fly, you can edit with a almost-light editor like Krita.
And if you're not into Firefox (waterfox being outdated), try https://github.com/Eloston/ungoogled-chromium instead of Google Chromium (which is bloated with spyware).
Cheers