Cool link to this: Advanced Technology
These are the best kind of ooparts because they have been documented, often photographed, and examined by experts:
"Spark plug" in a geode. In 1961, the owners of a gift shop in Olancha, Calif. found a fossil-encrusted geode in the Coso Mountains. When one of the owners cut the geode in half with a diamond saw, however, he found an object inside that was obviously artificial. The object had a metal core surrounded by layers of a ceramic-like material and a hexagonal wooden sleeve. When X-rayed, the object seemed to resemble a modern spark plug or some other electronic component. Yet it had been completely encased in a geode that was covered with fossils estimated to be 500,000 years old.
Very old nail. In 1851, The Illinois Springfield Republican reported that a businessman named Hiram de Witt found a fist-sized chunk of auriferous quartz while on a trip to California. When it accidentally slipped from his hands, it split open, and out fell a cut-iron nail. The quartz was about 1 million years old.
Gold thread among the rock. The Times of London reported in 1844 that workmen quarrying stone near the River Tweed in Scotland found a piece of gold thread embedded in the rock eight feet below ground level.
Chain in coal. In 1891, Mrs. S. W. Culp, of Morrisonville, Ill. was fragmenting coal into smaller pieces for her kitchen stove when she noticed a chain stuck in the coal. The chain measured about 10 inches long and was later found to be made of eight-carat gold, and described as being "of antique and quaint workmanship." According to the Morrisonville Times of June 11, investigators concluded that the chain had not simply been accidentally dropped in with the coal, since some of the coal still clung to the chain, while the part that had separated from it still bore the impression of where the chain had been encased.
Ancient modern tools. While quarrying limestone in 1786, workers came to a bed of sand about 50 feet below ground level. In the layer of sand, however, they found the stumps of stone pillars and fragments of half-worked rock. Digging further, they found coins, the petrified wooden handles of hammers, and pieces of other petrified wooden tools. The sand in which the discovery was made was beneath a layer of limestone dated at 300 million years old.
Mysterious vase. In June, 1851, Scientific American reprinted a report from the Boston Transcript about how a metallic vase, found in two parts, was dynamited out of solid rock 15 feet below the surface in Dorchester, Mass. The bell-shaped vase (see photo), measuring 4-1/2 inches high and 6-1/2 inches at the base, was composed of a zinc and silver alloy. On the sides were figures of flowers in bouquet arrangements, inlaid with pure silver. The estimated age of the rock out of which it came: 100,000 years.
Too-old screw. In 1865, a two-inch metal screw was discovered in a piece of feldspar unearthed from the Abbey Mine in Treasure City, Nev. The screw had long ago oxidized, but its form - particularly the shape of its threads - could be clearly seen in the feldspar. The stone was calculated to be 21 million years in age.
Ancient nanotechnology. In 1991-1993, gold prospectors on the Narada river on the eastern side of the Ural mountains in Russia found unusual, mostly spiral-shaped objects, the smallest measuring about 1/10,000th of an inch! The objects are composed of copper and the rare metals tungsten and molybdenum. Tests showed the objects to be between 20,000 and 318,000 years old.
There is more at the link - maps and whatnot.