CGTextures - Database of jpg textures, works great for texturing in Sketchup as well as in Photoshop. A free account allows you to download 15 downloads a day.
Flyingarchitect - V-Ray (Rhino 5) textures which include bump and displacement maps. Though the textures are created for V-Ray, you can use just the diffuse images in your 3D model and in Photoshop if needed.
Free Stock Textures - A database of exterior and plant textures.
Texturez - Eclectic database of images from food to sky in high quality JPGs.