A whole bunch of Simple Tools

KiCAD

Tools Description
PCB Outline Creator Used to rectangles with rounded corners for KiCAD PCB editor. Mainly targeted at creating PCB outlines.
Library Downloader Extract KiCAD files from SamacSys library (e.g. Mouser download)

LTSpice

Tools Description
MultiParameter Sweep Create commands to step multiple parameters at once in LTSpice, running a simulation for all possible combination of parameters.

General

Tools Description
DC-DC Converter Designer Simple design tool for DC-DC converters
Analog Filter Designer A tool to simulitate different filters with ngspice right in the browser