I'm a native English speaker and I used to be reasonably proficient in German (three years of it in high school and another semester in college). I can't speak it very well now, but I still occasionally dream in German. I really should bone up on it but for some reason I'm really fascinated with French right now and just bought an instant immersion French course for my Mac. I also managed to pick up a smattering of Latin and Greek during college, not enough to speak the languages but enough to get me by during my ancient history classes.
I think the best language class I ever took was when I was in ninth grade. My school had a really cool one-year program called "Foreign Language Experience". We took four weeks of Spanish, four weeks of French, four weeks of German, and three weeks of Italian, along with a week or so of Portuguese, a week of Swedish, and a week of Japanese. The focus was on mastering basic communication skills that a traveler would need in that country, but for that short period of time we were immersed in whatever language we were studying at the time. I didn't realize how much of it really sunk in until I went to Mexico a few years ago and was still able to get by. Most useful class I ever took.
We just don't put enough of a premium on learning other languages here in the US, which I find sad; it really opens up your world view. My fiancee and I have decided that whatever else our future children do, we want to make sure they're fluent in at least one language besides English.