I remember i saw quite a number of planes but only could identify 2 while i was cruising at a quite high attitude, one was a UA B747 on my way to Singapore on SQ, couldn't really get a glimpse of the exact model as the plane was just like a bullet!
However this August on SQ B744 from SIN to HKG, got my second glimpse of a Japan Airlines MD-11 J-Bird around 25,000 metres (While my SQ flight was around 30,000 metres and above)!Was really surprised as i really got to look very clearly at the J-Bird and it was on the left side of the SQ plane, should be heading for Singapore or either Kuala Lumour according to its course.And it looked so close!Also it was my very first time i could look at a plane so carefully and patiently (Though it lasted only for...5 seconds while cruising at such a high speed!A Never-Can-Forget-Experience!
Share your experiences, i'm sure it'd really be unforgettable!At least for me...it really makes me feel so lucky to get another glimpse of another plane flying at a cruising height.Afterall...the sky's so big!

Best Regards