I definitely believe that there is intelligent life out there.
I also believe we've been visited by extraterrestrial life albeit thousands of years ago.
The sheer number of other planets in the universe makes extra-terrestrial life almost a mathematical certainty. It's not certain, but the probability is close to it.
Not only that, but if you think about how old the universe is, and the fact that humans have only been around for thousands of years, and how much we have evolved and how much technology we have developed in such a short period of time, there must be life forms that are faaaaar more advanced than us. The universe is billions of years old, imagine what our technology will look like 20 years from now. Imagine 100 years from now. Imagine 1,000,000 years from now.
If you look through history, there are plenty of things that suggest that E.T's have in fact landed on Earth before. I'm by no means saying that it is definitive, but I think it's happened. I really like documentaries about this sort of topic. Some favorites are the series "The Universe" on the history channel or "Ancient Aliens". One episode of Ancient Aliens is all about old structures and I highly recommend it to any one who likes this sort of thing.