MATLAB (mathworks.com)
Scilab | Scilab
Online circuit simulator & schematic editor - CircuitLab
CPUlator Computer System Simulator (01xz.net)
GDB online Debugger | Compiler - Code, Compile, Run, Debug online C, C++ (onlinegdb.com)
KiCad EDA - Schematic Capture & PCB Design Software
Saleae Logic Analyzers
Digi XCTU — Download and Install the Configuration Platform for XBee/RF Solutions | Digi International