If you need to support multiple users with one computer, you are probably looking at a terminal server. Unlike point to point products such as Timbuktu, where what the user sees accurately reflects what the computer is displaying, terminal servers create a virtual session for each user, spreading the power of one computer to many clients.
The market leader for these products is Citrix, which has clients available for virtually every popular operating system including even MS-DOS, and one that is based on Java. Citrix's flagship product is called Metaframe, and uses a protocol called Independent Client Architecture, or ICA. Citrix products can be used on devices that lack a hard disk; this configuration can lead to lower costs and excellent manageability. However, the more users that need to be supported, the beefier the server must be. Furthermore, while ICA works well with most productivity applications, it is not well suited to multimedia applications.
Microsoft has its own version of Terminal Services available for Windows NT and 2000 that uses its own protocol, dubbed RDP (Remote Desktop Protocol). Using Windows Terminal Services, you can view a full Windows desktop on a PocketPC! Terminal Services has other features such as restricting the applications to which a remote user has access. While Microsoft doesn't support non-Windows clients, third parties have developed clients for Mac OS and Linux.