I have a few to recommend...
Firefox - Stop giving an ad network all of your data on a silver platter.
Affinity Photo - Good photo editing software with perpetual licensing.
digiKam - FOSS photo organizing software
Strawberry Music Player - A fork of a fork of amaroK, good music player!
VLC - Watch any video file.
Kodi - Consume your media library, in style!
OpenRA - Play the original Command & Conquer: Tiberian Dawn and Red Alert as well as Dune 2000 on modern hardware/software for free.
Unreal Tournament 2004 - I have bought this game three times, the original CD release on 6 discs, Steam and GOG. This is to my mind the best arena shooter ever, the original CD release even came with an official Linux installer.