The 9 Billion Names of God
Ruby, 46

?A.upto(?M*9){|s|s[/(.)\1{3}|[N-Z]/]||puts(s)}


My original, similar solution was longer and wrong (it output base13 numbers, which isn't quite all of them due to leading zeroes), but I'll leave it here because it got votes anyway.

1.upto(13**9){|i|(s=i.to_s 13)[/(.)\1{3}/]||puts(s)}