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Recent large galaxy surveys conducted with NASA's Hubble Space Telescope Advanced Camera for Surveys (ACS) have provided unprecedented legacy datasets, which allow astronomers to charter the evolution of galaxies over a large fraction of the age of the Universe. The Galaxies and Cosmos Explorer Tool (GCET; http://www.as.utexas.edu/gcet/) is an online web-based tool that allows the general public and students to actively engage in this exciting adventure through quantitative analyses of HST images from the Galaxy Evolution from Morphology and SEDs (GEMS) survey, one of the widest-area galaxy surveys conducted in two filters with ACS to date. The tool allows users to surf the vast cosmos and access ACS images of over 8,000 galaxies over the last eight billion years. For galaxies of interest, users can measure the size, determine the lookback time for concordance cosmology, perform morphological classification on images at two rest-frame wavelengths, and gauge the different stellar populations information, such as coordinates and redshift, of each galaxy into Excel spreadsheets for further analysis. The celestial coordinates can be used to extract further multiwavelength data from existing archives and upcoming virtual observatories. For undergraduate classes, more advanced IDL or C-based analyses that employ the full samples, can be combined with the visualization capabilities of GCET in order to explore the nature of interesting objects, such as the most massive galaxies, starbursting systems, interacting and merging galaxies. GCET provides a powerful tool for discovery learning in undergraduate science and introductory classes, as well as high schools. We thank Dario Landazuri and Chris Wilkinson for their invaluable help on getting GCET online. We thank Marco Barden, Boris Haeussler, and our other GEMS collaborators. We acknowledge support from NASA grants NAG5-13063 and NASA NNG 06GB99G, NSF grant AST-0607748, and the Faculty And Student Teams for Technology (~FAST Tex) award from the University of Texas Division of Instructional Innovation and Assessment (DIIA).