What is the alternative for Teamviewer?
There is Teamviewer it-self.
Otherwise, there is Open-source alternative. Look at some client for RDP protocol, or VNC protocol.
Teamviewer works on Ubuntu too. download for the 32 bit version or the 64 bit version. So if you like that as remote desktop software you can still use it ;)
But if you really want an alternative I suggest to take a look at VNC. But the server side as the client side are easy to get working.
I'd try VNC as mentioned above or ScreenConnect.