Mac to Windows, use the above mentioned Microsoft Remote Desktop Connection, or CoRD
Mac to Mac use Screen Sharing, it is based on VNC, but Apple's client and server when talking to each other is better than generic VNC. If you are connecting Mac to Mac over the internet, then use the Back-to-My-Mac feature in iCloud. On the same LAN Screen Sharing is easy.
TeamViewer.com is good at remote connections across the internet, and it is cross platform.
LogMeIn is OK, especially if going from Windows via Internet Explorer as it then uses DirectX enhancements to make the client fast. Going from a Mac via Safari to a LogMeIn system is OK at best, but not all that great (TeamViewer.com is better - personal opinion).
If you have to go from Mac to Unix/Linux systems, then look at Chicken (formally Chicken of the VNC), or JollysFastVNC. These VNC clients will play nicer with non-Apple VNC servers than Apples Screen Sharing VNC client.
If you have to go from Windows to Mac, then you need a VNC client, and you may or may not experience problems with the Mac VNC server, as it does not always play nice with 3rd party VNC clients. If you have the need, and TeamViewer.com or LogMeIn.com does not satisfy your needs, then you might look at the Vine Server (aka OSXvnc) as it does play nice with 3rd party VNC clients.
There is nothing magical about the Apple Remote Desktop with respect to screen sharing. As you have figured out it is for managing a class room or office of Mac, besides that it does not offer that much for a single user (or even a family where there are only a few systems).