Internet traffic from "always on" devices and bandwidth hungry applications are creating congestion on both fixed and mobile Internet service provider (ISP) networks. In response, ISPs have introduced "data caps", or quotas on the amount of data that a customer can send during a billing cycle. As ubiquitous, "always on" Internet devices and applications become commonplace, we must provide better information and tools to help users manage these caps. We present uCap, a data cap management tool that helps users monitor and control data usage from Internet-connected devices in their homes. We evaluate uCap through both a deployment of 21 households in three countries and indepth interviews from ten of these homes. To our knowledge, uCap is the first tool of its kind. We present the design and evaluation of uCap, study the behavior of users in the face of usage caps, and offer recommendations for the design of ubiquitous computing applications that must operate under data caps.