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