From now until April 1, Sony is offering $10 credit for every $50 spent at the PlayStation Store.
It’s worth noting that that you only need to spend $50 to get the $10 back—which means you can buy a few things that add up to $50+ or just one game that’s worth $50 or more. It’s also worth noting that you won’t get that $10 (or $20 if you spend $100, and so on and so forth) until the end April. From Sony’s own post about the promotion:
Starting February 26 and throughout the month of March, we’ll be offering you the chance to get $10 back for EVERY $50 you spend using your SEN Wallet! This means that if you spent $100 you’d receive $20 back at the end in April.
There are quite a few AAA titles that get day 1 digital releases coming up, though (MLB The Show 13, Army of Two: The Devil’s Cartel, BioShock Infinite), so finding something worth spending the money own to build up some PSN credit shouldn’t be too hard for anyone interested in doing so.