LogMeIn is a remote management applications that is available in different versions, depending on what features you need, such as remote management of a single desktop, entire networks, file transfer etc. There is a free versions available, with somewhat limited capabilities.

GoToMyPC is another widely used service for remote access or management of a PC.

Dameware is an enterprise class solution to remote management of any number of systems running any version of Windows.

VNC is a free remote management tool that can be used on a LAN or over a WAN. However, many versions are not encrypted and should therefore only be used with in a VPN if used to connect using the Internet. There are numerous versions available.

UltraVNC has a "Single Click" option. With this configuration you create a small custom file, less than 200kb, which you make available to, or e-mail to, your clients. It contains your site IP information, your custom company logo, and custom connection client. The client clicks on the file and it starts a connection which  approve when you receive the notification. You can then take control of the remote machine to service, or train the remote user. Nothing is installed on the client machine, and once closed you cannot connect again, until the user clicks on the file once more. If you want to be able to initiate the session at any time without user intervention, this is not a good option, but clients like it as it gives them the security of knowing you have to be "invited". No firewall reconfiguration at the client site is necessary, and the application/s are free.