Riemann-Stieltjes integration
If you've ever worked with Stieltjes integrals, specifically the Riemann-Stieltjes integral, and - like me - wondered if there's any intuitive interpretation to them like there is for the standard Riemann integral, then wonder no more, the answer is yes, as this lovely article (Bullock, A Geometric Interpretation of the Riemann-Stieltjes Integral, Am. Math. Mon 95:5 (May 1988), 448-455) quite literally illustrates. Give it a read! (It's free on JSTOR.)