Project Great: General Relativity Einstein/Essen Anniversary Test (2005)

A little excerpt:

According to Einstein, fast-moving clocks run slow (special relativity), and high-elevation clocks run fast (general relativity). Clocks that run fast gain time, so given our high elevation and how long we stayed, the prediction was that these clocks would gain about 22 nanoseconds. This, not because the clocks were moving (they were in a parked minivan), but simply because the clocks experienced a lower gravitational field by being 5400 feet above sea level for two days.

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The plot show the clocks aren't perfect (even these excellent atomic clocks vary randomly at the nanosecond level) but they each undergo positive time dilation and the average (black line) time jump in the three clocks while up on the mountain for two days is around 23 nanoseconds. Not bad for a first attempt.