dd and netcat

You want to make an image of /dev/sda on boxA over the network.

1.  Boot boxA using a liveCD
2.  On boxB: nc -l 1010  | dd of=file.img  // This will now sit and wait for incoming data and write it to the image file.
3.  On boxA: dd if=/dev/sda | nc 192.168.0.1 1010

Once the transfer has finished, dd will display information about the amount of data that has been written. netcat won't automatically exit, so upon seeing this information, you simply need to Ctrl+C to quit.