• KeepassXC [password locker]
• SyncThing [keep some dirs synced between computers]
• Thunderbird [Check 4 email accounts simultaneously]
• PikaBackup [Relatively easy disk backup utility]
• FSearch [Fast searcher of all files on my computer. Like Search Everything for Windows but worse in many ways]
• AudioBookshelf [Podcast server]

I use all of those pretty regularly. Honorable mention on iOS is a program called Is It Snappy? which helps me measure input lag. It doesn't collect any data or run ads (rare trait on a phone app). I actually made a purchasing decision with the help of this thing to correctly conclude that the Nintendo Switch Pro Controller had a noticeable input delay (enough to make me return it). The funny thing about that was if I just looked up spreadsheets others have done I would have seen that same conclusion there, too, without having to go through the effort of buying it myself :P.