Vincent Danen recommends installing CoRD for OS X, which allows you to remotely access PCs using the Microsoft Remote Desktop Protocol and control Linux desktops as well.
Mac OS X is very capable when it comes to controlling remote computers. If the other computers are Macs, then using the built-in Screen Sharing for systems connected locally works very well, and using screen sharing in iChat works great for remote users. If you want to control other systems, Screen Sharing will also work with VNC, as will a number of other OS X VNC clients.
But what if the remote system is a Windows computer? Yes, you can install a VNC server on the remote system, but Windows also has the ability to use its own built-in screen sharing which can be more powerful than VNC: Microsoft's Remote Desktop Protocol (RDP), also known as Terminal Services.
Unfortunately, OS X does not support RDP out-of-the-box. There is a solution, however, and that is with CoRD, which is an RDP client built specifically for OS X.
When CoRD is installed, fire it up, and you will be presented with a very basic blank window with a server list. The server list can be used to create "bookmarks" to various RDP-enabled systems. A Quick Connect field in the toolbar allows you to enter in the hostname of the system you wish to connect to.
To add a bookmarked server, click the + button at the bottom of the server pane (which can also be hidden, if you so desire). An inspector window will pop up where you can fill in information for the connection: the address, username, optional domain, and label that will be displayed in the server bookmark list. You can also set session preferences: the size of the screen, color depth, and various audio/performance options that can be tweaked.
CoRD is a very simple and very efficient open source, remote desktop client. The downside to the RDP protocol is that it is only available in Windows Vista or 7 business edition and higher, or Windows XP professional. If you fancy using it to remotely control a Linux desktop as well, you can do so by installing the xrdp package; many distributions like Fedora will include xdrp, so it is easy enough to install.
If you need, or want, to use Microsoft's Remote Desktop Protocol from a Mac, CoRD will do the trick. With it, OS X can be used to remotely control most other operating systems whether they rely on RDP or VNC.