Something that administrators deal with quite a bit is trying to connect to a machine using remote desktop (mstsc) and getting our favorite message:
Boy this messagewill get you angry, damn squatters. pre-Vista/LH you could always give the /console switch a go and see if the console session is available:
mstsc /console /v:<server>
Even if the console session is being used you can still kick the person off if you feel like it.
The other option is to query who is connected and (if you have the rights and don't care whats going on in their session) boot off whomever you don't want connected to free yourself a slot:
query session /SERVER:<server>
reset session /Server:<server> <id>
And in real practice, goodbye squatter2.
The query.exe and reset.exe commands live in System32 so they should come with the OS. Links to the syntax of those commands below.