When studying traditional painting techniques a while back, I became obsessed with creating digital swatch sets of various real media, particularly oils and acrylics. I've included most of them here for anyone else with an interest.
For an idea of how to sort through the colors and pick the most useful, see my post on choosing real paint colors for painting digitally.
Daler-Rowney Cryla Acrylics
(Golden updated their website a while back, leading me to create a new set of swatches. I actually prefer the old set however, so I've included it here as well.)
Old Holland Oils
Williamsburg Handmade Oils
Winsor & Newton Artist Oils
Winsor & Newton Artisan Oils
Winsor & Newton Winton Oils
Richeson Casein Paints
[Download Richeson Casein]
Winsor & Newton Cotman Watercolors
Photoshop Real Paint Swatches
(This swatch set comes with Photoshop; I've included it here for use in different programs, such as Manga Studio. These colors are more saturated than your typical real paints.)
Prismacolor Soft Core Colored Pencils