Dropbox is a cross-platform cloud storage and syncing tool that starts with 2 GB of space for every user, though that amount can be upped in various different ways – for example by recommending it to friends. If you got an Android device, however, there's now another way to add up to 5 GB of free Dropbox space, for a total of 7 GB.
In the latest experimental version of the Dropbox app for Android, there's a new feature called Camera Upload that automatically uploads all photos and videos taken with your smartphone. Upon uploading the first picture, you'll get 500 MB additional storage, and after that, every 500 MB of uploaded data using that feature will yield you another 500 MB storage expansion. So, by uploading a total of 4.5 GB, you'll add 5 GB of Dropbox space – of course, you can delete those files afterwards, if you want.
What's the catch, you're asking? Well, you cannot simply upload anything you like. You really have to use the new Camera Upload features to get the bonus space. Plus, you'll need to install an experimental version of the Dropbox app that may or may not be stable.
For the sake of completeness, you also get the additional space through the experimental desktop client, using the same feature.