My friend Paschou Constantin - aka The French Monkey - creates some of the most amazing products that are tailored towards helping artists focus on creating with ease and speed. I personally use his products and totally recommend them. Here's a list of some of the products I personally use, but make sure to check out his website for the complete roster.
This is the full collection of Greyscale maps from The French Monkey, and if you get the full collection you get them for $170 - that's 40% off!
Greyscale Kits - 8K Resolution
Really good high resolution and tileable. I use these all the time for details like scratches and smudges on my materials.
The kits include:
Grunge Details - $35
Organic Details - $35
Pebbles - $35
Scratches - $35
Smudges - $35
Greyscale Maps
Another great resource for detailing your materials. Perfect for making tech and organic details.
These include:
Brush Maps - $15
Grunge Maps - $15
Modular Maps - $35
Retina - $15
Techit Maps - $15
Voltaic - $15
Landscapes
The French Monkey even created some awesome 8K resolution packs for creating terrains.
These include:
Valley Kit - $35
Ruin Kit - $35
Macro Kit - $35