My wife and I are a fan of keeping things a mystery, so we waited (and are waiting with our second) to find out the gender.
But, according to some research, you can find out as soon as 14 weeks now!
Check out these mom's testimonies and recommendations...
http://www.babycenter.com/400_when-is-the-earliest-you-can-find-out-the-sex_2070466_420.bc