Finally found something really fun to do during Computer Graphics class: deviantART's excellent feature, muro! The digital art creation includes several brushes, color pickers, layers, transparency, and now allows you to save your work and return to it later (if you have an account). The platform also supports Wacom tablets=awesome! Spent probably about 2 hours total on this.
Check it out at http://muro.deviantart.com/.