I think this may be a new Leopard (10.5) feature, but perhaps I just never noticed it: if you close your LCD display (sleep your computer) during a file transfer, the transfer will resume after the machine is woken up.

For example, say you’re transferring a 5GB file from your Macbook to your Mac Pro on the same network. Halfway through the file transfer, you accidently shut your LCD display and the computer goes to sleep.

Do you have to start the transfer over? No! Open the LCD display and wake up your computer — it will automatically rejoin your network and immediately resume the file transfer from where it was stopped!

The only caveat here is if your computer can’t rejoin your network. If, after shutting your display, you move to another network or loose your WiFi signal, the transfer will give up and produce an error about 10 seconds after waking up.