About 18-20 weeks with my son, my first. I had actually been feeling movement for awhile, but I didn't know what it was. Then, the day after I stripped some wallpaper in the kitchen (and yes, my doctor had a couple words about that), I feel horrible abdominal pain. I had strained my muscles, which were loosening to begin with, but they put me on a fetal monitor for a bit to make sure everything was okay. We kept hearing these huge "whoosh" sounds, and a nurse came in and said, "Wow, he is really kicking in there. Are you feeling that?" And I realized I was!
With my daughter, I felt movement earlier, probably about 14 weeks. I was at the computer doing school work (I was still teaching), and I felt it. I remember just stopping and holding my stomach, tearing up, and hoping like mad it would happen again.
Haven't felt anything yet with this one. Assume I will in the next month or so.