Glossolalia is when you speak in a "language" that no one on earth (including yourself) can understand. (The idea is that God can.) Xenoglossy is when you speak in an actual language that you have no way of knowing. Both of these are usually called "speaking in tongues." The second one (Xenoglossy) is certainly more impressive. This is what the apostles (in the book of the Acts) reportedly did. They could preach to people who spoke other languages, as if they had a Babel Fish for the mouth. That's why Xenoglossy is often more specifically called "the gift of tongues," and why you'll be hard-pressed to actually find it. Plain old speaking in tongues, however, is fairly common. You may have done it yourself.
The second chapter of the Acts describes an event that takes place on the day of Pentecost (a day that marks the giving of the Ten Commandments) during which the apostles are filled with the Holy Ghost (or Holy Spirit) that causes them to speak in other languages. People of several nationalities (who all lived in Jerusalem) were confused to hear Galileans speaking their native language. In addition to speaking in tongues, they also appeared drunk to the observers.
A similar thing happened in Los Angeles in 1906 during the Azusa Street Revival, which lasted (more or less) until 1915 and spawned the Pentecostal (and Charismatic) movement. There may not have been any Xenoglossy, but there was Glossolalia, and certainly what looked like drunkenness. Though mostly black, there were also whites at the revival, as well as a combination of rich and poor, and they were all "holy rolling" together. "A disgraceful intermingling of the races," a local newspaper wrote.
Okay, so what do I think about all this?
First, I've never seen Xenoglossy, actually speaking in an existing language that one hasn't learned. I don't know anyone who has. I have been in church services during which members have attempted interpretation of tongues (another gift of the spirit), but the fact that these tongues were typically translated into King James style English makes one dubious. The gift of interpretation is stuck in 1611?
As for speaking in tongues, speaking sounds that has the characteristics of language: there's certainly something to it. And let's keep it simple for now; let's remove God and see what's left. What happens when someone speaks in what seems like another language, if they can get to that state?
The University of Pennsylvania (as reported in The New York Times) found that, while speaking in tongues, activity in the parts of the brain that have to do with self-control (and language itself) are reduced while emotional centers are heightened. So something exists, that's not in the form of a drug, that makes you feel pretty high (as if you're speaking to God himself, if that gives you an idea of the sensation). Something exists, that's not in the form of alcohol, that allows you to lose control of yourself so that you have a drunk feeling--but without a hangover in the morning, loss of memory, vomiting, etc. In fact, you're plenty conscious the entire time (as those parts of the brain are active). Something exists that makes you feel euphoric while it's happening and then better in the morning. Who wouldn't want this?
This takes me back to Azusa Street. No wonder people of all kinds were flocking together. What looked like a disgraceful intermingling of the races to the bigots of the early 1900s felt like a beautiful communion of humanity that ignored race and class. I've seen this first-hand, too, in churches. Reserved, potentially-racist white men are suddenly hugging all over black men (eliminating, apparently, any potential homophobia as well). It never ended in a bar fight, and it caused actual racists to rethink their opinions: "Last night, while we were speaking in tongues together, he felt like a brother to me," etc. So, removing God for now, you have something like the best effects of ecstasy and alcohol, without any of the negative effects, and the added bonus of instant brotherly love.
Let's think of phenomena that, to me, seem similar: laughing and crying. Both are similarly weird. One of them even causes liquid to drip from our eyes. And both are beneficial. What if we didn't laugh? "Laughter is the best medicine," anyone? Freud said laughter releases psychic energy and tension. And crying. I suppose we'd just shrivel up and die without tears. Maybe there's some third, misunderstood, often-missing emotional element that goes along with the outbursts associated with comedy and drama.
I didn't mean, by the way, for this post to become some advertisement for speaking in tongues. For one, I don't think it's something that most could just start doing. And those who speak in tongues might be offended that I'm taking away the "gift from God" aspect and placing tongues in the realm of the hippie-dippy or faddish (something that could catch on as the new yoga). I'm really just wanting to talk about it as a potentially natural phenomena, to perhaps defend it, or at least to look at it from the point of view of those who experience it, to explain it in a non-religious way to those looking on at these confusing and "disgraceful" displays of emotion.
So if you've ever felt sorry for someone who you think has deluded themselves into thinking they're talking to God, then don't feel sorry. Feel jealous.