First, foremost and without whom none of this would exist... Crate Entertainment.
A very small indie company originally started by and consisting of just Medierra, ex Titan Quest dev. Currently up to about 10 people and without doubt one of the very best companies I have ever had the pleasure of being involved with during alpha/beta/early release/whatever the current popular term is. Regular updates, and regularly participating in the forums, listening to player feedback and adjusting the game to suit while staying true to their vision of what Grim Dawn should be as well as constantly producing far more content all the time than originally planned.
The ultimate skill calculator and planner. Started, maintained and kept regularly updated by Stormcaller. Easy planning of any build up to any level including ones not yet in the game including stat points as well as just skill points. Includes a saving feature giving you a link to keep or easily share builds on the forums etc.
Rumours are there is a Devotion planner in the works by Stormcaller as well. Check their site and the Grim Dawn forums for further info.
The definitive collection of every single item in the Grim Dawn world that can be acquired by the player. This is a stunning collection started, maintained and regularly updated by Shalie. A clean and easy to use site, with full filtering and selection choices to allow you narrow your search down. All the items come with their accompanying artwork, full stats (with ranges), including any bonus stats and with components the ability to combine different amounts of partial components to see the change of stats as more/less are combined and the range of completion bonuses.
Grim Dawn Wiki.
Info to follow shortly.