Pitot tube - measure airflow speed
Example uses: Find areas of separated and attached flow, measure thickness of boundary layers, assess airflow direction for setting wing angles
Equipment: model aircraft pitot tube, sensitive Magnehelic gauge
Complexity: easy to do and interpretation of results straightforward
Comment: allows assessment of attached / separated flow on undertray out of sight of wool tufts
Cost: $10 (pitot) + $50 (gauge)
(I must credit F1 aerodynamicist Willem Toet for bringing these pitot tubes to my attention. He mentioned them to me when describing some low cost data logging equipment being used in student-based racing cars. I didn't go for the data logging equipment, but loved the pitot tubes!)