Not only do I believe that aliens have already visited our planet, I believe that we are a product of those visitations. There are numerous recordings of alien visitation throughout history, and for those who believe in Creationism some of that evidence is in the Bible. God supposedly brought forth Noah's Flood as a direct result of his Creation cross-breeding with a race of giants (...and in those days there were giants on this Earth). A race of giants could very easily have come from a planet with a smaller gravity well than our own, and the nearest contender for this is Mars.
Mars is interesting in itself. There is some evidence to suggest that Mars has suffered a cataclysmic impact in its very recent past (cosmically speaking), possibly around the time that the Sphinx was carved out of the bedrock on the Giza Plateau in Egypt, and current evidence in some quarters dates the Sphinx to have been made around 8000BC and possibly as early as 10500BC. There was no civilization on Earth that we know of at that point in history which would have had the skills and the means to create something as monumental as the Sphinx, so who built it and where did they come from? There is also the small matter of an alleged "face" and a pyramid on the surface of Mars in area known as Cydonia. I have seen the photographs taken by Viking 1 in 1976 and the later digitally doctored (sorry, enhanced) pictures taken by Global Surveyor in 1998, and despite NASA's best efforts at debunking the face I'm sorry to say that I am keeping the book open.
It is worth pointing out that the ancient gods of the Egyptian and Greek civilizations were depicted as giants, which could also point to them having come from a different planet. There is also the Lid of Palenque depicting what looks very much like an astronaut at the controls of a spacecraft. We must also question how it is that the Dogon tribe have such an expansive knowledge of the star Sirius. They have no means of observing Sirius with anything other than the naked eye, yet they knew it to be a binary star. There are fingerprints all over this planet to suggest that aliens have been coming here for a very long time. There is almost a bottomless pit of books written on the subject of alien visitation and UFO encounters and several of these books are in my own library. If I had to recommend one to read it would probably be The Stargate Conspiracy (ISBN 0-7515-2996-6) by Lynn Picknett and Clive Prince. I don't usually do adverts but buy it and read it, it's a bit of a mind blower. Alternatively, try Fingerprints Of The Gods by Graham Hancock ( sorry, don't have the ISBN to hand). It's a Little gentler on the brain and less likely to keep you awake at night.