I got some useful tools/sites for you guys. Feel free to suggest others.
Image editing software
Designed for simulating the effects of natural mediums. Basic version is free.
For working with Raster images and similar to Photoshop. Free and open source with lots of useful plugins. Easy to use if you're familiar with Photoshop.
For those looking to work with vectors this provides an alternative to Adobe Illustrator. Easy to use, free and open source.
Helpful for creating color pallets. Give it a color and it will suggest six complementary colors, all web safe.
This is particularly useful if you're designing an OC.
Free with no download needed.