Contact - TUBE FLARING TUBE FLARE ORFS FLARE 37 FLARE 45 FLARE

The only thing you get after the credits is an achievement. And no, there is no hidden ending in Limbo. If you beat the game without dying once you'll get another secret egg (and is required to get 111% in the game, but there's no achievement for that), but otherwise there's no secret ending.