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!
Google Docs Publisher tries to fix the garbled formatting when trying to publish a Google Docs document to the web. Simply publish the document as usual, then paste the URL into Google Doc Publisher. You will get a new URL pointing to the correctly formatted document.
Gravit is a web-based version of vector design tools such as Adobe Illustrator. It includes the usual pen, line and polygon tools, as well as object alignment/grouping, layers etc. Files can be exported to PNG, JPG or SVG. These file formats can be imported as well.