PencilSheep is an online image editor that provides essential functions such as layers, filters, layer styles, adjustment layers, masks, selections, brushes, undo/redo, raw file import etc. It supports various different color formats with different ranges and bit depths (up to 32 bit per channel / 256 bit per pixel HDR).
Vectr is an online vector graphics editor, like a simpler version of Adobe Illustrator that runs in the web browser. You can save your work to the cloud, or export it to PNG, JPG or SVG files. It is also great for real-time collaboration, as each design gets its own unique URL.
Smallpdf offers many browser-based PDF processing tools, including compressing, merging or unlocking PDF files, as well as conversion from/to JPG/Excel/Word etc. The tools are free to use, but a paid upgrade gets you additional benefits such as an ad-free experience and multiple file processing.
SandwichPDF lets you convert a scanned PDF document into a fully searchable PDF. The PDF will be converted to text using OCR (Optical Character Recognition) and the resulting text will be included as searchable text layer in the PDF. The original layout of the PDF will be retained.
Circuit Simulator is built using HTML5 and will run in any modern browser on a reasonable fast machine. You can hook up all the usual components of a circuit including resisters, capacitors, voltage sources, wires etc. and visualize how it works without building anything physical. A great way to experiment!