Other predictions of general relativity, like the bending of starlight by the sun, the advance of the perihelion of Mercury, and time dilation due to the strength of the gravitational field, have become standard observations for many years. Gravitational waves, on the other hand, while they were indirectly detected back in 1974 (for which a Nobel Prize in Physics was awarded in 1993 to Russell Hulse and Joseph Taylor), have not been "seen" directly.
Hulse and Taylor discovered the first binary pulsar (the discovery paper is here), which is a pair of neutron stars orbiting each other . As any two masses orbit each other, they emit gravitational waves, similar to the electromagnetic waves that are emitted when two electric charges orbit each other. A very nice exposition of the physics of pulsars and gravitational waves can be found in this Scientific American article by Taylor.
In 1911, when Ernest Rutherford (with help from Hans Geiger and Ernest Marsden) discovered that the structure of atoms could be described as a massive positively charged nucleus, surrounded by very light, negatively charged electrons. It was assumed that these electrons orbited the nucleus similar to the planets orbiting the sun (hence this model was called Rutherford's planetary model of the atom. However, Maxwell's theory of electromagnetics, known since 1865, predicted that such a structure would be unstable. For example, take the hydrogen atom, with a single proton for a nucleus and a single orbiting electron. Maxwell's theory predicted that the electron would emit electromagnetic waves, lose energy, and spiral into the proton in about 10-11 s (one one-hundredth of a nanosecond!). Hence, atoms should not be stable, and people should not exist. Obviously we do, so Niels Bohr, in 1913, postulated that the atom was governed by the laws of quantum mechanics, which meant that the electron could exist in a particular energy state and be "stationary," and not radiate electromagnetic waves.
Gravity (as described by general relativity) appears not to be quantum mechanical (or at least we haven't yet discovered the quantum mechanical version, although physicists are trying), and orbiting masses do lose energy by emitting gravitational waves. For example, the Earth radiates about 200 watts of power as it orbits the sun. This points out the difficulty with detecting such waves: they are extremely weak! They are weaker than electromagnetic waves for two reasons. First, the gravitational force is about a billion billion billion billion times weaker than the electromagnetic force (10-36 times weaker). Second, there is only one kind of mass (positive), while there are two types of electric charge (positive and negative). In technical terms, this means that electromagnetic radiation can be of the "dipole" form, which the strongest gravitational radiation is "quadrupole." Quadrupole radiation is weaker than dipole radiation by a factor of (v/c)2, where v is the speed of the orbiting object and c is the speed of light. For an electron in a hydrogen atom v/c = 0.007, and for the Earth orbiting the sun, v = 29 km/s, which is about 0.0001 times the speed of light.
This means that the vibrations in the masses that are used to detect these gravitational waves are about one one-hundredth the size of a proton (0.01 fm)! It was an amazing technical feat that was achieved.
Here is a plot of the detector positions as a function of time from both the Washington and Louisiana sites.
