Varying definitions of a meter, and events related thereto:

1791 (The end of the French Revolution): The meter is defined as 10-7 times the distance between the North Pole and the Equator along a meridian through Paris. However, the value representing the circumference of the Earth used in the calculation is wrong.

1799: The (incorrect) relationship between the Earth's circumference and the meter is discarded. The new meter is defined as the length of the prototype - a platinum bar.

1866: Use of the metric system is legalized (!) in the U.S.

1875: The International Bureau of Weights and Measures is established. Twenty countries sign the Treaty of the Meter and join the International Metric Convention to impose some fucking order.

1889: A new prototype meter bar is made - this time with an X-shaped cross-section. The distance between two scratches in a platinum bar stored in a vault next to its metric pal, the kilogram, at the International Bureau of Weights and Measures denotes the meter. Every member country in the International Metric Convention recieve two copies of the prototype.

1960: The Eleventh General Conference on Weights and Measures redefined the International Standard of Length as 1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86. Other, secondary, definitions using Mercury 198 and Cadmium 114 were also accepted by the General Conference.

1980: Yet another secondary definition, this time using an iodine-stabilized helium/neon laser, is accepted. This type of laser has a wavelength uncertainty of 1/1010.

From 1983 and on: The meter is defined as the distance travelled by light in a vacuum in the time interval of 1/299,792,458 second, thus making distance a function of time and providing you with a way of reliably measuring distance should you ever run out of Mercury 198 or Cadmium 114...

You've come a long way, little meter.
And now... sources!
Oolong, who pointed out that the lightspeed definition actually was accepted in 1983.