DaConLab
Electronics design tools that run in your browser. Pick one to get started.
Gerbcilla

3D Gerber and CAD viewer. Open fabrication files, ODB++ or IPC-2581 and inspect the board as real copper, FR-4, mask and silk, down to a single via or a cross-section.
Smith.chart

RF design on a Smith chart: impedance matching, transmission-line calculations and PCB antenna design.
Bommer

BOM health check for KiCad and Altium. Flags EOL, NRND and single-source parts, checks stock for your build and finds price breaks.
CSpice

SPICE-lite circuit playground. Drag parts onto a schematic, wire them up and watch the oscilloscope update live as you turn the knobs.
Heatsinker

Heat sink design and thermal simulation. Place heat sources, shape the fins and see junction temperatures and margins.