This seemed harder to Google for than you'd think.
Imagine you want a desktop on Ubuntu Linux on an AWS EC2 instance, and you'd like to access it from OS X. The sanest thing is to follow these instructions on installing kubuntu-desktop and tightvnc on the Ubuntu instance [with more info here], and the "Screen Sharing" app built into OS X. Access the Ubuntu instance with "[ip address]:590[x]" where [x] is the X display number that tightvnc attaches to, generally 1 (so it would be "[ip address]:5901"). Don't forget to set up a security group to allow the VNC port(s).