There are two authors who have contributed mightily to the idea that earth was visiten by ancient astronauts: Erich von Daniken and Arthur C. Clarke. Now, I hear you scoff in front of your computer monitor. Von Daniken, of course, even Sitchin, perhaps, but Clarke? He writes science fiction.
Oh, ye of little memory. In his groundbreaking book, 2001: A Space Odyssey, Clarke posits that human evolution was spurred on through the sudden leaps made possible by alien seed technology which jump starts evolutionary changes, ending eventually as a "star child." This is the true source of New Age alien contact theories. Clarke wrote "Sentinel," the origin of the 2001 novel, in 1948. Von Daniken came up with his archeology revisionism in the late sixties.
The idea of "paleocontact," or contact with an alien species in prehistoric times is a serious field, graced with the attention of notable scientists like Carl Sagan, I.S. Shklovskii and Hermann Oberth. There are several reasons why people consider this a legitimate area of study.
Von Daniken is not one of those reasons. Von Daniken's book Chariot of the Gods, or Sitchin's The War of Gods and Men, for instance, are compilations of pseudo science and deliberate half-truths. Their basic premise, and what I object to very strongly, is that just because people lived a long time ago, or had not developed a high level of technology, they could not make incredible achievements. The Egyptians built monuments in stone that rival anything man has ever created. Unless, of course you consider that the Meso-Americans did too. Oh, and the Chinese. And the Celts. And the Amerinds. And the West Africans. And the East Africans. Every culture that finds sufficient leisure time and creates a basic hierarchical structure sufficient to allow for slavery and forced conscript labor, builds huge earthworks. Every culture that is fundamentally agriculture develops a working calendar based on seasons, which means that they automatically match star patterns.
Von Daniken looks at a tiny part of a stone sarcophagus and claims that it looks like a spaceman in a spaceship. He neglects to mention that the spaceship is a dragon, that there are ritual symbols that he ignores and that the whole thing is old news to researchers. He sees great works of stone and claims that human beings are too stupid to have developed the relatively simple techniques for moving the stones or manufacturing monuments of any complexity or exactness. I find this at least un-humanist in its insulting denigration of the capabilities of our fore-bearers, and at worst down right racist.
Here are some of those things that suggest alien contact, but which reall are examples of human ingenuity. The problem with linking any of these feats with ancient astronauts is the sheer number of examples of their efforts at learning how to do this kind of construction. And in the case of the truly herculean examples, like the pyramids of Giza, the sheer scale of the burial yards for the workers who built the monuments.
Nazca lines: duplicated by researchers using stone age tools.
Pyramids at Giza: duplicated at a small scale by a dozen researchers using bronze tools. Also, evidence now shows a learning curve of building crude and small pyramids before attempting large scale projects.
Castle of Zimbabwe: Verifiably created with bronze tools.
Megalithic Towers at  Baalbek, Lebanon: Discovered to be lower than quarry, the towering stones did not need to be lifted.
Moai of Easter Island: Smaller examples of these large stone sculptures waing over 6 tons were moved by researchers with simple implements that the islanders use.
A lot of paleocontact believers see ancient astronauts in human art works. Especially religious texts often contain examples that could be interpreted as alien contact. Space craft, airplanes, helicopters can be seen in Egyptian murals. But for every example, there is a simpler and more rational explanation. Airplanes are stylized birds. Spaceships are angelic creatures.
A piece of genuinely puzzling paleocontact is the existence of the Dogon people. In 1931 this primitive people were discovered to have a knowledge of astrophysics way beyond what even modern science had a handle on, but mostly they had knowledge of astronomical features that one cannot know without basic optics and a high level of mathematics. This is, perhaps, the most significant example of paleocontact possible.
The Dogon knew, for instance, that Sirius had a companion star, Sirius B. This companion is so faint that it cannot be made out even with most ordinary telescopes. They had a knowledge of the rings of Saturn and the moons of Jupiter. For a people living in stone age conditions, this was remarkable. Criticisms of the anthropologists who made these discoveries have been unconvincing and mostly attributable to skepticism and bitter rivalry, and not to any evidence. Speculations about contamination remain only conjecture. But the Ddon have had this Sirius cult for centuries, and their mythology also claims that there is a planet around the Sirius system that the founders of their tribe came from.
This knowledge beyond their abilities to know are not conclusive by any means, but it does provide evidence that is intriguing.