This paper is the source of the “largest triangle three buckets” (LTTB) algorithm that’s become popular in the past few years.
Another interesting one out there is ASAP. While LTTB tries to maintain the “envelope” of a signal, choosing local extreme values instead of averaging, ASAP is all about averaging. But it uses a kind of adaptive moving average that tries to select a window that will smooth out high-frequency noise and periodic components that are visually distracting, while stopping before discarding “interesting” variation. It seems to work reasonably often.