Not sure if still available, but Gaea 1.0 had a free tier for 1024 maps: https://quadspinner.com/compare
It's a terrain tool, node-based.
Another one (for terrain) that i've just found that seems to be free even commercially (don't use the plant catalog stuff though, that's not for commercial use, make your own with it)! https://www.bentley.com/software/e-on-software-free-downloads/
Source/mini tutorial that i've found for it: