Windows
Windows Server 2003, 2008, Tablet PC Editions, and Windows Vista Ultimate, Enterprise and Business editions come with Microsoft's Microsoft Management Console, Windows Registry Editor and various command-line utilities that may be used to administer a remote machine. One form of remote administration is remote desktop software, and Windows includes a Remote Desktop Connection client for this purpose.
Windows XP comes with a built-in remote administration tools called Remote Assistance and Remote Desktop, these are restricted versions of the Windows Server 2003 Terminal Services meant only for helping users and remote administration. With a simple hack/patch (derived from the beta version of Windows XP) it's possible to "unlock" XP to a fully featured Terminal Server.
Windows Server 2003 comes with built-in remote administration tools, including a web application and a simplified version of Terminal Services designed for Remote administration.
Active Directory and other features found in Microsoft's Windows NT Domains allow for remote administration of computers that are members of the domain, including editing the Registry and modifying system services and access to the system's "Computer Management" Microsoft Management Console snap-in.
Some third-party remote desktop software programs perform the same job.
Back Orifice, whilst commonly used as a script kiddie tool, claims to be a remote-administration and system management tool. Critics have previously stated that the capabilities of the software require a very loose definition of what "administration" entails.
Remote Server Administration Tools for Windows 7 enables IT administrators to manage roles and features that are installed on remote computers that are running Windows Server 2008 R2
Non-Windows
VNC can be used for remote administration of computers, however it is increasingly being used as an equivalent of Terminal Services and Remote Desktop Protocol for multi-user environments.
Linux, UNIX and BSD support remote administration via remote login, typically via SSH (The use of the Telnet protocol has been phased out due to security concerns). X-server connection forwarding, often tunneled over SSH for security, allows GUI programs to be used remotely. VNC is also available for these operating systems.
Apple Remote Desktop provides Macintosh users with remote administration capabilities.
NX and its Google fork Neatx are free graphical Desktop sharing solutions for the X Window System with Clients for different platforms like Linux, Windows and Mac OS X. There is also an enhanced commercial version of NX Server available.