Dropbox, one of the most watched companies in Silicon Valley, officially announced on Monday that it raised an astounding $250 million in a Series B round led by Index Ventures, reportedly valuing the virtual file cabinet company at a whopping $4 billion.  This massive round stands in contrast to the first bit of money raised — about  $7 million –  from early investors including Sequoia Partners, Accel Partners, and Hadi and Ali Partovi.*