Dropbox will give you 2Gb free storage space when you register an account. You can get 250Mb of free space for Each user joining through your invitation (The user who joins also will get additional 250Mb space).
to get 250 Mg visit Getting Started Page complete any 5 of the 6 quests.
To get 768MB of additional space you need to have a twitter and a Facebook accounts.
Visit the Free page and finish all the steps for each steps you will get 128Mb of free space.