Accessibility
I have a few ideas for our good friend the multimeter. (I'm only thinking about the multimeter because I spent the whole week soldering stuff to perforated boards.)
1. Multimeters usually have audible feedback for continuity tests. The advantage of this is that you can test your circuit without having to look away. Of course, for hearing-impaired individuals (or just folks in a load room), this advantage doesn't really imply. My multimeter would include a bright LED that lit up when continuity was achieved, or perhaps the unit itself would light up - something that you could catch in the corner of your eye.
2. Let's replace the posts with clips! If you could clip one terminal onto your circuit, you could hold the circuit in position with one hand while moving the opposite terminal in the other. This would also make using the multimeter easier for individuals with reduced dexterity.
3. Automatic mode switching. If continuity isn't detected, switch to testing for resistance or voltage, perhaps displaying both simultaneously. This would reduce dependence on interface elements like switches, buttons, and knobs that can be difficult to use (on my multimeter at least).
4. Tactile and/or audible feedback, not just for continuity but also for resistance, voltage, amperage. Map vibration or pitch to any of these variables. More modalities means less dependence on any one modality.