Home made marking knife: extremely low-level (i.e. razor-like) thin, narrow single-bevel angled blade @ one end, a sharp needle point (protected by wine corks) @ the other.
Blindman's calipers. Used to have beautifully cheap ($5) but now unfortunately unreadable set of Spanish verniers for 40 years or so. Modern digitals always need spare batteries as they never switch fully off!
Locking plier jaws multitool: Vise Grip/Schrade "Toolbox" (pref), or as an alternative a Kershaw A100 or (least fav) Leatherman Crunch. Multitool pliers are useless unless they lock! A Spyderench is a useful but eccentric alternative twist on the "fixed/adjustable jaw" multitool concept. Multitools are better on the belt than in the toolbox in my opinion.
Cheap Chinese alloy small roofing square for marking, measuring, right angle fixing & as an x-cut power saw guide.
Internal/external measuring tape: for doorways, windows & recesses. Metric only; dual measures are foolish, & won't allow space for internal/external scales.
Laser disto (not so inexpensive, but a def. must-have for my everyday kit. Love the pythagorean, square & cubic volumetric measure functions. The ONLY reliable & accurate sole-operator measure.
Bahco ratchet screwdriver with bit caddy in the handle & switchable LED lights.
Voltstick: a literal lifesaver!
Disposable earplugs.
3/4" mason's chisel.
70's vintage Stanley low angle block plane with wide lands (60 1/2A) for trimming, arrises, easing drawers, shooting miters etc.
Bahco/Sandvik Ergo folding pullsaw.
