From here: http://superuser.com/questions/125636/can-a-mac-machine-be-used-by-multiuser-at-same-time
If this answers your question, consider deleting it as it is a duplicate.
Yes, it is possible using Vine Server (OSXvnc). I never used it, but the creators claim:
[..] in Tiger (Mac OS 10.4) all those
desktops can be accessed
simultaneously using Vine Server
(OSXvnc). This allows multiple users
to be logged in, each using his or her
own desktop on the same Mac.
All they need is a computer or PDA
running a VNC viewer to access Vine
Server (OSXvnc). Now you have a use
for all those old PC's!
Each user MUST be logged in using Fast User Switching. If you reboot the computer you will need to go and log-in each user with Fast User Switching to enable access via VNC.
And using the very same software simultaneously might yield problems (like when temporary files are not stored in a user area), or might break the license agreements.
I don't know if this still works in Leopard and Snow Leopard. (But a recent post on Server Fault might indicate it's tested on a recent version of OS X, hence Snow Leopard?)