You can simply open iChat and open the video window. Also, the Video menu has a function to take a snapshot.
I used to monitor my house with this method while I was away on vacation. I figured that it the computer was still there and on, the place hadn’t burned down or been completely ransacked.
After my house was recently burglarized, I got more serious about security. I paid US$50 for http://www.securityspy.com/ which uses the iSight to take time lapse or motion activated images and movies. I have it set to upload them to an offsite server in case the computer is taken. The best part is that it emails me with some snaps right away.
This software has a built-in server for monitoring and settings. I save the VNC sessions for other computer operations I need.