Slay the Spire finishes its run in Early Access on January 23, but that impending deadline hasn't stopped the developers from pushing out a major new feature in the first patch of the year. Patch 54, released on Thursday, includes Steam Workshop support for modders who want to make their own cards or other changes. All you have to do to enable mod support is choose "Play with mods" from a new launcher when you start the game. Presumably this will disable Steam Achievements, but we haven't verified for ourselves yet (and achievements aren't mentioned in the patch notes).
Modders have already been tinkering with Slay the Spire for awhile, but Workshop support will naturally make mods easier to install, and should make for a more vibrant mod scene. As of this writing, there are already 41 mods in the Workshop, including an API to make modding easier, content packs that add cards and relics to the game, and several new characters. Here are some to check out if you've already shown the spire who's boss a few times.
Replay the Spire and Hubris both add a whole bunch of relics, dozens of cards, new bosses, events, and run modifiers. Replay the Spire is the bigger of the two, and also adds some elite units. But either one should give you a significantly expanded possibility pool for any given run.
The Construct: One of several new mod characters, The Construct seems especially fleshed out, with a pool of more than 100 cards and a detailed description on its mod page: "The Construct's main strength is versatility - cards with the Cycle keyword will automatically discard themselves and redraw if you can't make use of them. Combine this with Mode Shift cards that allow you to alter your Strength and Dexterity on the fly, and you can ensure you always have the right tools for the job." It even includes its own bundled "mini expansion" that adds a new risk/reward overheating mechanic.
The Disciple: A character "themed after the Time Eater" with "four central themes: Card Retention, Intent Shifting, Card Transforming, and Temporary Relic Cycling." The Disciple already has quite a bit of positive feedback.
Mimic Mod: If you thought the chests treasure rooms were too safe and rewarding, this is the mod for you. Now they might be mimics.
Colored Map: Pretty simple, but you can change the colors of icons on the progress map to make them easier to distinguish. Or just ugly, if that's your jam.
RatMod: A mod for the character The Silent, which adds more cards and new mechanics: bleeding, silvered, combo, and echo. Combo and silvered sound the most interesting: combo cards do more damage based on how many cards were played before them, and silver cards force the enemy to cleanse and deal damage based on stacked silver cards. Mostly, though, I like this mod for its picture, which I'm pretty sure is an opossum.