Software related links

Most of them free and open-source

Audacity
Ocenaudio; not open-source but a great audio editor alternative for Linux, Audacity is great but runs poorly on my machine...

Stellarium
Space Engine; impressive virtual universe with accurate rendering of many things (planets surface etc.)

Algodoo; quite fun 2D physics sandbox, bit old but the physics engine is still strong ! (linear constraint solver + SPH for fluids), used it a few times to model mechanical / fluidics computers
The Powder Toy; same but pixels based
Ray Optics Simulation; see gallery for many cool examples

Colobot (also Ceebot); old and i don't really agree with the language design but this was huge fun back then, although totally different (a RTS) i also liked Planet Blupi (1997; Eggbert) of the Blupi series by the same developer when younger, has a 90s cute goofy aura with a fun (and naive) settings, this also relate to the Smaky computers !

Soulseek; file-sharing network to exchange music, my go to when i don't find some albums elsewhere

wxHexEditor; useful hexadecimal editor to view / edit big files (or partitions, devices etc.), ImHex is also great for old-school platforms (has a web version and also many disassembly formats etc.)
localhost.run; expose local services to the internet quickly using SSH tunnels (free, no signup, secure, easily sharable URL after connection)
IEEE 754 floating point numbers explorer
online animated GIF tools (useful for TIC-80 records etc.), crop, resize, cut, change gif speed...

Lutris; open gaming platform with no fuss install of games + multiple sources library (GOG, Humble Bundle, Epic Games Store, EA, Ubisoft, custom etc.), especially love the easy install of some open source remake of old games such as Caesar III (with official data coming from e.g. GOG)

VDO.Ninja : P2P streaming / sharing of diverse sources in the browser (privacy, low-latency, free, no downloads, no data collection, no sign-in), quite useful to quickly share heavy media stuff to friends, also allow recording