Whereas server-side programs are essential to maintain databases and run data analysis pipelines and simulations, client-side web-based computing tools are also important as they allow users to access, visualize and analyze the content delivered to their devices on-the-fly and interactively. This article reviews the best-established tools for in-browser plugin-less programming, including JavaScript as used in HTML5 as well as related web technologies. Through examples based on JavaScript libraries, web applets, and even full web apps, either alone or coupled to each other, the article puts on the spotlight the potential of these technologies for carrying out numerical calculations, text processing and mining, retrieval and analysis of data through queries to online databases and web services, effective visualization of data including 3D visualization and even virtual and augmented reality; all of them in the browser at relatively low programming effort, with applications in cheminformatics, structural biology, biophysics, and genomics, among other molecular sciences.