As far as I can tell, the distribution Yahoo is offering is just the vanilla Hadoop, but with Yahoo's patches on top of it. Yahoo is very involved in Hadoop's development (the project's founder is now employed by them), so a lot of their patches get incorporated back into Hadoop's source tree. Most of the changes Yahoo made are just performance/stability patches that haven't been incorporated into an official release yet. You could probably get the same distribution just by grabbing SVN trunk.