The big address is likely MtGox, they often transfer funds through a key that holds around 500K BTC.
Actually ... I also thought that that 500K BTC address was Gox's initially, but
some people disagree. I forgot the thread where this was discussed.
I agree. I also do not believe it is GOX. Gox cold storage is much more 'detached' from actual user addresses than what is apparent here from this particular addy.