Scitable

"Scitable is a free science library and personal learning tool brought to you by Nature Publishing Group, the world's leading publisher of science.
Scitable currently concentrates on genetics and cell biology, which include the topics of evolution, gene expression, and the rich complexity of cellular processes shared by living organisms. Scitable also offers resources for the budding scientist, with advice about effective science communication and career paths."

http://www.nature.com/scitable