This is a list of all the open source software I have come across and use frequently to semi frequently. There will likely be some overlap with stuff everyone has already posted.

Photography and Image manipulation

Darktable → RAW photo processing
GIMP→ Photoshop alternative
Krita → Digital painting (have only used it a bit, but I hear it's good)
Inkscape → Vector Graphics
Automatic1111 → Diffusion model AI toolkit (mostly Stable Diffusion but also has extensions for other diffusion based models like OpenAI's Shap-E)

3D modeling and Printing

Blender → 3D Modeling, sculpting, 2D animation, compositing all rolled together (simply one of the best examples of FOSS)
Meshroom → Photogrammetry
PrusaSlicer → 3D printing slicer based on Slic3r

Video editing and Processing

Kdenlive → Genuinely good video editor
FFMPEG → Command line media toolkit (very complex but also works on android through Termux)
Instant NeRF → Neural Radiance Fields, think photoscan to a 3D representation (not meant to make 3D meshes unfortunately)

Misc

Calibre → E-book management
Serge → Self hosted Local LLM's made a bit easier to deal with
Firefox → Web browser

FOSS I'm excited for

DragGAN → Manipulate images by intuitively dragging, more on this here and here (official code being released this month but there are already projects based on the paper with working examples)
CoDi → "Composable Diffusion" Any2Any conversion Txt2Vid, Vid2Audio, Audio+Txt2Img, whatever
Neuralangelo → Promises to be NeRF's for 3D models (don't know if it will be FOSS but I'm hopeful)