Has anyone ever seen a ghost?
The short answer is yes, many people have seen ghosts, but the majority of what people think are ghosts or paranormal activity can usually be explained. Until you actually see one, it's difficult to understand what they look like.
Ghosts can take on many forms and shapes, from wisps of vapor, glowing orbs (not the orbs you see on pictures - most of those aren't orbs), shadow entities (the most common form) and full bodied apparitions, either transparent or full life color apparitions.
As one Investigator in a small group of active Paranormal Investigators (we have much of the same equipment, tools and methods that TAPS does and uses) most of what we see can be explained as natural occurences. However, I can guarantee you that once you see a ghost, no matter what form it is, you will know then and there that they are real, they exist, and you'll get a chill down your spine like you've never had before.
While I've not had the privilege of seeing a full apparition yet, I have seen Shadow entities (dark masses that have a rough human shape but not distinct enough) manifest directly in front of myself and others, and have been touched by one. What is said about feeling cold is true - it's like being touched by cold air. One member of our group has actually had an NDE - he drowned but was revived in time. It is that reason we believe he is routinely singled out by the entities that we encounter. He is always being touched, poked, etc. One entity we encountered we didn't realize was there, but suspected because of recorder activity where nothing was present and no sound was being made. My friend entered the room I was in and his eyes immediately teared up. At the time we did not know what it was - he chalked it up to an allergic reaction or something (even though he has no allergies, it was the most logical assumption at the time). Later, it happened 2 more times, and those times were when the entity had manifested in front of us and was interacting with us. Later when we reviewed the IR camera footage, we clearly saw the entity move across the camera field of view not long after his eyes began tearing up.
Many people won't believe in ghosts because they're afraid of the unknown, have watched too many stupid movies, or their religious beliefs get the best of them. However, many people who have lost friends and loved ones have experienced their spirits returning to their home immediately after death or feeling their presence after they've died. My mother experienced this last year when my grandfather passed away - I was with him when he died, and it took a few minutes for me to compose myself before I called and notified her. She told me she felt his spirit go by her in his old home where she was staying, and thought he might have passed on. When she told me the time it happened, it was right after he passed away.
Whether you believe or don't believe is irrelevant - the question is whether or not someone has ever seen a ghost. For those of us who have, the answer is yes. For those who haven't, it's a shame, but ghosts aren't that prevalent, and you usually don't see them except at a fleeting glance. That's why it's important that we as investigators try to gather as much visual and audio evidence as possible, and that's not easy given the target. Remember that ghosts were people once, and they don't necessarily like to be disturbed, as many of us don't.
No one can ever say that "ghosts don't exist" - that's ridiculous. I still have a book from growing up in the '60's that talks about whether or not we'll ever visit the moon. Nothing is impossible, and nothing is certain either. Belief is not required to see a ghost, because even for those who don't believe, seeing one will make them believers in a heartbeat - if they don't have a heart attack first.