1.

Kerberos plus LDAP - This is a lower-level option where you set up Linux to use Active Directory's underlying protocols yourself.

2.

Samba - Samba is the defacto standard for joining a Linux machine to a Windows domain.

3.

Likewise - Likewise is basically Samba-in-a-box; easier to set up than a full-fledged Samba installation (the GUI should do it for you in under an hour, including time to read its docs), but correspondingly less control. Likewise Open is free of charge; Likewise Enterprise adds features like managing Linux machines via Group Policy.

4.

Microsoft Windows Services for Unix includes options for serving usernames to Linux / UNIX via NIS and for synchronizing passwords to Linux / UNIX machines. You'd use this if you wanted to do everything possible from Windows or if you had an existing Linux / UNIX infrastructure you wanted to tie to Windows; for most environments, though, one of the other solutions would be better.

5.

PowerBroker