HTML

• Can I Include? Check if you can nest one HTML element in another
• SVG Favicon Maker

Colors

• Sorted CSS colors

SVG

• SVG Path Editor
• SVMOMG SVGO's Missing GUI, clean and compress SVG's in a pinch.
• SVG Path Visualizer Explains step by step how a path is composed.
• SVG Path Commander
• SVG2JSX

Data

• DrawData.xyz Draw mock data and download it as CSV or JSON

Maps

• geojson.io Draw a geojson on a map. Check out Placemark a full-fledges map editor in the browser by the same author.
• mapshaper Tools for editing Shapefile, GeoJSON, TopoJSON and CSV files

Images

• Squoosh Optimize images

JavaScript

• Operator Lookup Learn about JavaScript operators