The Sony PlayStation Network
offers a free $10 credit
when you spend $50 in the PlayStation Store
from now through March. That's the best such offer we've seen from PlayStation Network. (We saw a free $20 credit with $100 spent in October. In this case you'll get a credit for every $50 spent; the previous offer was limited to a single credit.) You'll receive the credit before April 12.
Of note, the Sony team has stated repeatedly that purchasing a 1-year PlayStation Plus membership for $49.99 will count as spending $50 and qualify for the bonus. Update: Furthermore, 1-year Playstation Plus memberships come with an additional three months of service. Offer ends March 4.