The most popular move is 5...Qc7, but my favorite has always been 5...a6. I've played this position so many times with Black that I've lost count.
1. e2e4  1... c7c5  2. g1f3  2... e7e6  3. d2d4  3... c5d4  4. f3d4  4... b8c6  5. b1c3 