for multi OS support in server and client, there is ultra VNC, it offers many bandwidth tweaks and is fairly easy to setup, on the flip side its ease of setup means the only form of security is the lone password you set up. And you cant set up rule based logins (for vnc) nore havemore then one logon. Also because of this, ANYONE (provided ports are set to allow for it) can attempt a connect and get the logon prompt. This is generaly bad if you try and set this up to be used over the net, for like when you are away from home.
Win xp, 2k server, and 2k3 allow for windows remote networking (almost any windows computer can download a client to access these though, winxp comes with the client built in). This is very close to VNC but alot better (but may not be dialup friendly). For one I have noticed it never exhibits visual artifacts which makes me think actual window data is being sent rather then just bit map iinformation. Also when you logon, you are loging in as a user, you can use any valid logon to the computer, you are restricted by that account, your desktop is unique so that anyone sitting at the coimputer does not actually see your remote desktop, and you can have multiple users (local and remote) loged in at the same time, each with their own unique desktop. You CAN't of course have the same user logged in more then once, so don't go logging in as admin if you boot the computer and are logged in localy as admin.

you can log back to the login screen and then access the computer remotly. I do this with my win2k server, it has no keyboard and boot to the logon screen, and i have all users logon to it over the network using their unique accounts.
Of course there are a few limitations, audio isn't full proof (MS desktop is ok, VNC dosnt support it), images may be compressed, and movies and games are out of the question on all of these.