I have a few that come to mind.
A buddy of mine wanted me to play Ace Combat 5, so he lent it to me. I didn't really think I'd care for it that much, since nearly everything I play has more fantasy than piloting a fighter jet, but it was pretty fun. I mean, not fun enough to seek out any of the other Ace Combat games or anything, but I enjoyed it.
Another time, a guy I worked with wanted to borrow something of mine and lent me Rainbow Six: Vegas, and for the same reasons as Ace Combat 5, I didn't expect to like it. But that was really fun, too. There wasn't a whole lot of story to get into (which is funny, since it has an author's name plastered on it, but I digress), but it was a fun, competent shooter. I even bought the sequel used several years back, but haven't touched it yet.
