Linux
We are comfortable with both MS Windows and Linux, with the realization that both have their place in the market.
Our favorite Linux distribution is Debian. Debian's social contract ensures that it will remain free and not controlled by any commercial body. The developer community remains strong and dedicated to providing high quality releases even if it takes longer to do so. Having used Debian since the Bo or Rexx days, we have run our fair share of apt-get and dpkg updates...
Of course, there are many different flavours of Linux. Fortunately, they all share many components: the Linux kernel, shells like BASH, web servers like Apache, programming languages, desktop environments, and databases.
Looking for Single Sign On (SSO) between Windows and Linux? We have done many Samba setups with NTLM and Kerberos. This allows for SSO not only for file sharing, but also authentication and authorization of other services such as Apache, Squid, SSH, and FTP.