Historically, if you wanted to use Active Directory to authenticate users on a UNIX box, you were pretty much limited to using LDAP. This works fine for some people, but it’s not particularly elegant – especially if you’re having to create users home directories all the time, which negates some of the point of centralising authentication to begin with.
I’m from a UNIX (mostly Linux) background, so I’m more at home using UNIX-alike platforms. That said, there’s a few things that Microsoft do that are particularly useful, and in my opinion AD is one of them (quiet at the back, there). Handily, there’s a project that can marry the two, and it goes by the name of Likewise.