Collaborative web browsing tasks occur frequently, such as one user showing another how to use a web site, several users working together on a search task, or even one user sending an interesting link to another user. Unfortunately, tools for browsing the web are commonly designed for a single user. PlayByPlay is a generalpurpose web collaboration tool that uses the communication model of instant messaging to support a variety of collaborative browsing tasks. PlayByPlay also supports collaborative browsing between mobile and desktop users, which we believe is useful for on-the-go scenarios. We conducted user studies of the desktop and mobile versions of PlayByPlay and found the system to be usable and effective. Author Keywords Collaboration, web, collaborative browsing, co-browsing, mobile, design, CoScripter ACM Classification Keywords H.4.m. Information systems applications: Miscellaneous.