I have used Photo Transfer in the past—prior to PhotoStream. It isn't free but it worked pretty well. There are free apps that claim to work but I've not tried any of them.
An iCloud account includes PhotoStream which just works. What more could you want?
The OP asked specifically about an iMac with OS X 10.6...
The PhotoStream requirements from the PhotoStream FAQ includes this line:
Mac with OS X Lion v10.7.5 or later and iPhoto 9.2.2 or Aperture 3.2.3 or later