"Active Directory Users and Computers" MMC snap-in for Windows 7?

For Windows Vista and Windows 7 you need to get the Remote Server Administration Tools (RSAT) - the Active Directory Users & Computers Snap-In is included in that pack. Download link: Remote Server Administration Tools for Windows 7.

Per Noalt's answer is correct. However, if you want the snap-in mentioned in the title (Users and Computers), you'll also have to run these commands at a command-line afterwards, as an Administrator:

dism /online /enable-feature /featurename:RemoteServerAdministrationTools-Roles-AD-DS
dism /online /enable-feature /featurename:RemoteServerAdministrationTools-Roles-AD-DS-SnapIns

Note - I had to run these in order for AD Users and Computers to show up, as they were disabled on my computer after the install. This might not be the case for all users.