When the Sonys and Samsungs of the world launch a new gadget, they go to CES. The startups? They head to SXSW Interactive. SXSWi was the coming-out party for modern essentials like Twitter and Foursquare. We look at the companies hoping to use this year's festival as a launching pad to Next Big Thing status—and let you know who seems the most likely to break through into your phone or bookmark tab.