It appears tangible evidence could exist that other races may be visiting this planet. This evidence comes from autopsies of supposed pilots and passengers of their vehicles. While not everyone will concur that the photos are indeed real, they are all worth viewing. If these photographs are real, then the evidence suggests there might be four different races, with perhaps a sub race.
Also see the page on crash-retrievals.
There are more photographs of course, but they belong to the class of photos that sit in the question basket.
This page relates to the last books in the New Earth Miniseries, and if you have read the miniseries you will know these four races. While the books are fiction, inspiration came from the above, be the photographs true or not.