Hi there Oblivion,
I imagine that a telnet server can be set up on pretty much any machine. I think on Windows XP professional, you can even enable it through administrative options, without installing anything. As kevlarman mentioned, on Linux (and cygwin), SSH has superseded telnet, I believe. Other software to look into would be puTTY, UltraVNC (or TightVNC), and Windows' "Remote Desktop Connection", which actually works very well and has a client for Macs.