Level Up! is an online community created by two Polish artists to help other artists to, well, level up, their game, as it were. They have both a website with a lot of resources ready for your use, and a YouTube station that has an ungodly amount of in-depth videos exploring every possible topic and artist you little heart may desire.
One of the things they do from time to time is over-paint. Basically an interested (presumably aspiring) artist will send in their artwork, and this Polish duo of rockstars will basically redo the artwork in real-time for the audience. While there is naturally some controversy with this approach; nevertheless it is highly educational to see how these two artists, who are hugely proficient with their craft, approach a piece, addressing any variety of concerns including composition, perspective, and color theory. It really is an education, and worth watching these 1- to 2-hour long sessions from start to finish.
If that were not enough, they have even established 3 permanent Google Hangouts for use by artists to use while doing artwork. While I have not leveraged this yet, it is an intriguing idea, especially for digital artists who are using a desktop where sharing a screen is feasible.