This $50 free Google adwords credit should be from Visa Business Network, (which offer free facebook advertising credit) or Visa related party whom co-operate with Google adwords to provide free google adwords credit upon sign up.

google visa

As long you have a visa credit card, and wish to promote your website or business online, get trageted traffic around the world. Here is a good place for you to start your Advertising

You can obtain the news at google service or directly at this Google Adwords sign up page.

Other then this offer, if you are Yahoo Small Business customer, you also can claim a $50 google adwords credit from there with Google Adword partnership with Yahoo Small Business