The first time we got a healthy dose of alternate reality was in season eight. One "What if?" question spirals us into a world where Lexie's a drug addict, Alex and Meredith are together, Cristina has bangs (the horror!), and Bailey's a huge nerd.
The second alternate-reality episode happened just recently! A patient causes Cristina to reevaluate what's important to her; we see alternate futures where she has children, and where Owen becomes an alcoholic.