Do you mean actual mods or QoL tools?
For mods Grimarillion is pretty good - lots of different masteries to choose from to make new class combos, you can ramp up the difficulty to insane levels if you want. Dawn of Masteries is similar, but is pretty new so still has bugs atm. If you like D2 then Reign of Terror is the one to go for; completely new maps recreating D2 in GD.
QoL - well that depends on what you're looking for. Grim Internals does all sorts of things; some of its features were added to the vanilla game recently, but there are still features that players like to use it for. Rainbow Filter allows you to see the difference between items. Grim Dawn Item Assistant and GDStash are both infinite storage systems; GDStash also allows character and item editing if that's your thing.
Have a look on the forum under Modding Projects and Utilities and Resources
https://forums.crateentertainment.com/c/grimdawn/modding-projects https://forums.crateentertainment.com/c/grimdawn/utilities-and-resources