Internet-based information and knowledge systems developed under the umbrella of geoinformatics have the potential to revolutionize traditional classroom practice. As the availability of computers and Internet connections in classrooms increases, the potential of harvesting such resources to advance teaching and learning provides unprecedented opportunities for both researchers and educators. Currently, numerous online resources are utilized in classrooms. This study focuses on such a resource, Discover Our Earth (http://www.discoverourearth.org), and evaluates its uses in classroom environments at the undergraduate and middle school levels. Discover Our Earth is designed around the theme of plate tectonics and covers topics such as earthquakes, volcanoes, topography, and sea level change. It is designed to promote hands-on experiences and inquiry-based learning. Users can access relevant data, mapping tools, and additional interactive virtual experiments from Web browsers without any additional required software installation. In order to fully understand the impact of technology tools in the classroom, it is critical that they be evaluated thoroughly. To evaluate the effectiveness of this online resource, we undertook both formative and summative mixed-method evaluations. Using a combination of quantitative (pretests and posttests) and qualitative (questionnaires and observations) methods in our evaluation design enabled us to evaluate the Discover Our Earth tools in our targeted user groups. We found that Discover Our Earth resources are more valuable and have significant advantages over using commercially available software. Formative evaluations in undergraduate classes helped us correct a series of user interface design issues at the early stages of software development. We also observed that Discover Our Earth resources enabled students to engage in inquiry and to develop their own understanding of plate tectonics through the exploration and visualization of the data sets. Summative tests conducted at middle schools showed that students paid more attention to interactive tools, and they continued to be engaged in learning activities even after the class was over.