The first place you want to visit is : https://www.dropbox.com/free ; You can get an additional 640 MBs just by connecting your Facebook and Twitter accounts and shooting out some tweets. Nothing too complicated and worth the 2 minutes it takes.
For those with more than a couple of minutes to spare and up for a neat challenge, Dropbox has created a very clever digital scavenger hunt (DropQuest 2011). Throughout the quest, you pick up MBs here and there until completing the challenge with an extra GB in your account. If you have the time, it’s actually a really neat challenge with a variety of puzzles and obstacles to get past. Very similar to many of the viral marketing campaigns you may have run across in the past.