Here is my list of tools I use:

Software:

• AutoHotKey: provides support for keyboard shortcuts for faster access to applications and typing symbols
• Inkscape: very handy to create illustrations and also taking PDF files apart e.g. schematic files or PCB layout files
• Word: write your specifications
• Excel: calculation tool for dimensioning of circuits, analysis tool for test results
• SMath Studio/(Mathcad): write your calculations in real formulas and see what gets calculated (cleaner than using Excel)
• Layout/Schematic viewer: in my company there is a home-grown tool that combines schematic, PCB layout, parts list, signal list and test point location and BOM synchronisation into a single, easy-to-use tool. If your desing tool does provide this: lucky one. 🙂
  Kicad has a Plugin that provides this functionality: InteractiveHtmlBom
• Test automation tool: a tool that is used to create test scenarios that are to be performed by the device under test. creates test steps, loops, sets test parameters and generates test reports from the gained results. can control not only the hardware under test but also remotely controllable test equipment like power supplies, multimeters, frequency generators and so on.
• FreeCAD: view step files, make mechanical mockups
• Programming Software for your programmable devices
• Data Loggers:
  • monitors and stores your data traffic of your device under test
  • monitors and stores measurements from your test equipment
• HxD: Hexeditor, for comparing data dumps from your system
• Version control (several different) for:
  • Assembly versions: keeps track of each assembly version BOM changes, firmware versions (where applicable), modifications
  • Documents: keeps track of each edit of your specifications and test reports or datasheets
  • Firmware: keeps track of your changes to your source files
  • EDA: keeps track of your changes to your schematic and PCB design
  • BOM: keeps track of your BOM changes between different assembly versions
  • Test Skripts: when did you change some parameter or sequence for your qualification test setup
• Material database: tracks your material including all PCB versions, components, mechanical parts, auxilary material. provides prices, stock, description, datasheet, order information, material status and preferably links to your Knowledge Database where necessary.
• Bug Tracker: keeps track of your defects and documents your solutions
• Knowledge Database (e.g. Wiki): helps you to remember things you researched once
  • some edge behaviour of some component
  • your assessment of some errata for your controller

Hardware:

Preferably all equipment has an interface that can be accessed via open, documented protocols. This makes test automation easy e.g. via python scripts.

Test Equipment:

• Oscilloscope: 4 channels recommended, minimum bandwidth 350 MHz, test probes with minimum 500 MHz bandwidth. Sample rate at least 10 times of bandwidth for accurate measurements.
• Multimeter: (bench or handhold) at least 0.5 % accuracy required, 4 digits readout.
• Bench power supply: with at least two seperate outputs >20 V and ≥ 3 A output capability. can preferably controlled remotely for test automation.
• Measurement data logger
• programming adapters for your programmable devices
• communication adapters for connection to your test equipment/DUT

Toolbox:

• Soldering station: Minimum power recommended: 75 W for fast heatup. My preferred supplier: JBC, for hobbyists Aifen provides similar specs
• side cutter
• pliers
• pincette
• set of small screwdrivers (flathead, Philips, torx, hex)
• set of normal size screwdrivers (all of the above standards + Pozidriv)
• power drill
• caliper
• band measure
• ruler