Authentication architecture on a Unix Network

Until now we always used authentication again local /etc/passwd file
for our Unix servers.
We have about 300 machines of different OS, Sun, HP, AIX and some Linux
We would like to centralize all this part of authentication.
We do not would like to use NIS,NIS+
All recommendations, advices, links are welcome.
What I'm interested is on a centralized architecture. As the principle
of ActiveDirectory on Windows domain
Thank you