r/cth was originally a subreddit for the podcast Chapo Trap House, but it eventually bloomed into a general-purpose leftist space. IIRC, the sub was around 160k members at its peak and had a distinct posting culture.
It was quarantined by Reddit for violent speech after calling for the death of slave owners and later banned for no particular reason at the same time as r/the_donald, presumably as a “both sides” sort of thing.
Hexbear is an instance formed mostly by former r/chapotraphouse users after that sub was banned from Reddit a few years ago. Hexbear used to run on a custom fork of Lemmy so that the community could add extra features that they wanted (like custom emoji) but it was recently ported back to mainline Lemmy after merging or reimplementing as many changes as possible.
Currently, Hexbear does not have federation enabled, and there is discussion about who to federate with or even whether to federate at all. The community is very active and self-sufficient and some members prefer the isolation.
Content-wise, it’s a leftist-focused instance. Some shitposts, some serious posts, and a lot of inside jokes.
Here’s one I know. Creates aesthetic wavy things.
https://tanck.nl/wallpaper/
It is an HTML/JS pair, so it would require some wrangling to get it to write to a file instead of an HTML canvas. Though it looks like someone translated it to Python with file output here. (I have not tested this.)