NASA's Kepler space telescope was designed to detect the very slight "dip" in starlight as an exoplanet transits its parent star. So far, the mission has identified over 2,300 exoplanetary candidates (i.e. signals that still need to be verified that they are actually exoplanets) in a small viewing area of the Milky Way.
In multi-planetary systems Transit Timing Variations (TTVs) may be detected in transiting exoplanets. Slight variations in their orbital timing can reveal the gravitational presence of a nearby world, even though it may not be visible.
In a handful of cases, Gravitational Microlensing is caused when a star passes in front of another, more distant star. The gravitational field of the closer star causes the light of the distant star to "bend" around it, much like a magnifying lens. A spike in brightness will be detected. Should there be any exoplanets orbiting the closer star, their gravitational presence will leave a signal imprinted in the microlensed light.
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There are other more refined techniques, but for me the real excitement will come when we can finally directly observe an exoplanet's atmosphere and surface detail.