drop.io is a well known, recently sold to Facebook. path.io changed to path.com boxcar.io is an awesome iPhone notifications app. pen.io is a new startup from t

drop.io is a well known, recently sold to Facebook. path.io changed to path.com boxcar.io is an awesome iPhone notifications app. pen.io is a new startup from the Launch Conference. Thats just off the top of my head... If you find any more please add them to this thread :)