To better promote their newest game, Playdom is offering Social City players a free Shopping District for reaching Level 5 in Market Street. The Shopping District is a Leisure type, takes up 9x9 tiles, and builds in one minute, netting you 530 XP. After it's built, it rakes in +7,500 Happiness Points. You can also sell it for 2,000 Coins.
Market Street, released back in July, is like the micro to Social City's macro. While Social City has you turning tiny SimCity-esque towns into large urban blocks, Market Street makes you into a small business owner of a goods store, where you can eventually build yourself into a giant retailer.