Box is a competitor of Dropbox, and essentially does the same thing.  See here for more info on the current promotion.  The offer is supposedly only good for LG or Sony Ericsson phones, but there's a work-around that appears to work on any Android phone (possibly tablets too?):

 

1) Search the Google Play Store (formally the Android Market) for the app called Droid of the Day (DOTD).

2) Download and install said app (it is free).

3) Open DOTD and select 'History'.

4) Scroll down to 'Box' from 3/7/2012.

5) Press 'Download'.  If it is your first time using the app, you'll have to read a short disclaimer and press 'OK I got it'.

6) Complete action using 'Play Store'.  This will take you to the Box page in the Play Store.

7) Press 'Download' to download and install the Box app.

Once installed, open the Box app and register a new account.

9) Done!  You should see a notification that you have received a free 50 GB account.  I tested this personally using my Motorola Droid 2 phone.

 

Now, it could very well be that steps 1-6 aren't necessary and that you can just go directly to the Box app's page on Google Play, download and install the app, and get the free 50 GB account.  However, I just tried it myself and these are the steps I followed.  So to play it safe, I would recommend all of the above steps.  DOTD seems like a pretty handy app anyway, so I think I'll keep it as well.

 

I believe the free account also comes with a file size upgrade to 100 MB.  So this probably won't work well for videos, but for music, photos, and documents it should work fine.  I believe the promotion ends on 3/21/2012 (or possibly sooner), so hop to it!

 

Edit: So I just realized that Box's 'desktop sync' feature is only available for business or better accounts (this free account is personal).  So basically you don't have a folder on your computer that always syncs like you do with DropBox.  The best use for this free 50 GB personal account is probably to extend your phone's music and photo library.  I think DropBox is more useful for business documents and collaborative work.  Just a heads-up.

only thing I didnt like about it when I did mine a few weeks ago, your limited to 100mb file sizes....

 

Myk

 

I did this a couple weeks ago.  You don't need to do anything but install the app and log in or create a new account.  The promo runs until March 23rd.  That being said, without desktop sync, I don't really see what I would use it for.