Alternatives to KDE Plasma
Use a full Linux desktop with configurable panels, widgets and window behavior. The listings below can replace it for an important use case. Each note says what changes if you switch.
The original
KDE Plasma
Use a full Linux desktop with configurable panels, widgets and window behavior.
Replacements
Listings that take over the same core job as KDE Plasma.
GNOME
Use a Linux desktop organized around an activities overview and workspaces.
GNOME organizes the desktop around an activities overview and workspaces instead of configurable panels, and its shell extensions must match the installed version.
Cinnamon
Use a Linux desktop with a traditional application menu, panel and system tray.
Cinnamon offers a traditional menu, panel and system tray with applets, desklets and extensions, though extensions may need updates for new releases.
Xfce
Use a modular desktop environment built around traditional panels and windows.
Xfce is a modular desktop with traditional panels and a settings manager, whose available version and components depend on your distribution.
COSMIC
Use System76's desktop environment with configurable workspaces and window tiling.
COSMIC is System76's desktop with configurable workspaces and built-in tiling and floating workflows, needing a supported distribution and graphics stack.
Similar software
Related functionality, not a direct replacement.
Openbox
Manage X11 windows with a configurable stacking window manager.
BunsenLabs Linux
A Debian-based distribution with a preconfigured, lightweight Openbox desktop, continuing CrunchBang Linux.
Tint2
Lightweight panel and taskbar for Linux and BSD window managers.