A lot of fans have compiled a bunch of different resources relating to Pathologic and its fandom. These are mainly collected from posts I've seen on social media as well as links I've seen passed around on Discord servers.
Classic HD Emotion Animations Chart - "the files of Pathologic Classic HD handle character expressions by assigning each NPC four or five animation scripts, which are stored under the name of the corresponding emotion. all characters’ files have a “Neutral” state, and while there’s a lot of overlap for emotions like “Smile” and “Fear”, many characters have unique ones!" This doc also has a few extra neat details in separate spreadsheets. (credit to rathologic on tumblr)
Pathologic Dialogue Trees - Here you can find every single line of dialogue in both Pathologic Classic HD and Pathologic 2 neatly organized in a branching format (just like in the game!) As of right now, it includes all the dialogue in Classic HD, Pathologic 2's Haruspex Route, and the Marble Nest DLC.
Steppe Language Dictionary - An organized and evolving fan-maintained dictionary of words/phrases used in the game's fictional Steppe language. There is a short introduction to the document explaining how the dictionary was made and how it is maintained.
A description of the Pathologic file format. - Exactly what it says on the tin. The author notes that they hope this page will help people interested in modding/translating the game. To my knowledge, this mainly applies to Classic HD; I'm unsure if this information helps with modding the original 2005 release.
Pathologic: Relapse Mod - An overhaul mod that spruces the game up a bit. They include updated textures and sounds, increased walking speed (matched with in-game time), and some slightly edited in-game text. I don't plan on frequently listing mods, but I thought this was kind of neat. This might potentially be a good introduction to new fans who want to play Pathologic Classic HD but are turned off by certain more-criticised aspects of the game.
Patho Classic File Unpacker - Again, exactly what it says on the tin. This program allows you to view the Pathologic game files in easily viewable formats, which is a helpful tool for anyone interested in modding the game. Admittedly, I haven't used this before, but my friend has a Tumblr post explaining how he used it to rip and edit the game textures.
Pathologic (2005) Website Archvive - An archive of the official Pathologic (2005) website. There are mainly updates about the game's progress, some commentary from the developers/artists and some extra neat little tidbits provided by Ice-Pick Lodge themselves.