Mobile devices such as cell-phones and PDAs present unique opportunities and challenges. Naïve transfer of applications from full-size computers often fails because the interaction becomes too cumbersome and challenging for the user. We address these opportunities and challenges by giving portable devices commonsense knowledge -- a large collection of simple facts about people and everyday life. We illustrate our approach with descriptions of several applications we have implemented for mobile devices using the Open Mind and ConceptNet resources. These include a dynamic phrasebook for tourists, an assistant for searching personal social networks, and a predictive typing aid that uses semantic information rather than statistics to suggest word completions.