This webapplication is a small javascript utility I've written to help me with visually generating the Python struct.pack() and struct.unpack() format strings. It also include a simple visual hex-dump debugging screen to see if the alignment is correct on my test data.
The main feature of this is to prevent manually going through format strings and count the sizes of the datatypes to try figure out at that offset a specific byte is. Most struct.pack and struct.unpack commands can be pasted into the format field straight from your python source and the data field accepts hex dumps and python strings as source.