Free accounts are 2GB, however you can do a couple of tasks to get more.

With few tricks I went to 18GB.

Could you elaborate?

I used a $100 Google Adwords coupon to advertise Dropbox on Google with my referral link. I did it to proof it could be done, not that I needed the extra space. Many others have done the same in the past http://www.lifehacker.com.au/2011/11/max-out-your-dropbox-re...

I won't speak for the parent, but, some people have been know to use Virtual Machine that pretend to be a "Friend" and take advantage of Dropbox's customer acquisition technique.

In mid 2013 I signed up 22 referred accounts in a VM box for a family member with a newly created Dropbox account. Three months later all 22 inactive accounts were removed, and the extra storage was gone too.

I did it in 2011 as well for my own account and still have the extra storage. I've since upgraded though. Even 15GB is peanuts today...

This works. They use your MAC address of your network adapter to differentiate referrals.

No they don't; I've referred multiple times from the same machine using different users.

You get space when linking your Dropbox with Twitter, and things like referring friends. If you participate in the yearly challenges, you get space for every step you complete successfully.

I do not know if it is still the case, but if you used the "Camera Upload" feature in the past you could get extra space at 500 MB for each 500 MB uploaded up to a certain capped amount. When the feature was in beta it was capped at 5GB extra, but before it went stable they lowered the cap to 3GB.