There are about 40 flightless species of birds living today. There are four key differences between birds that fly and those that can't:
1. Flightless birds have more feathers.
2. The wing bones of flightless birds are proportionally smaller to their body than other birds.
3. Flightless birds have a flat breastbone or a greatly reduced keel. Flying birds have a keel on their breastbone. The keel anchors the strong muscles that move a bird's wings during flight.
4. Flightless birds developed other survival and travel methods to fit their lifestyle. For example: Ostriches have powerful legs to run or defend themselves, Penguins are excellent swimmers, and Kakapos are nocturnal parrots with camouflage plumage.
List of recent flightless birds
(Flightless bird list from Wikipedia, the free encyclopedia)
Junin Flightless Grebe
Titicaca Flightless Grebe
Pelicaniformes (Pelicans, Cormorants, et al)
Magellanic Flightless Steamer Duck
Falkland Flightless Steamer Duck
White-headed Flightless Steamer Duck
Auckland Island Teal
Campbell Island Teal
Gruiformes (Cranes, Rails)
Woodford's Rail (probably flightless)
New Caledonian Rail
Lord Howe Woodhen
New Britain Rail
Roviana Rail ("flightless, or nearly so" [Taylor 1998])
Inaccessible Island Rail
Henderson Island Crake
New Guinea Flightless Rail
Samoan Wood Rail
Makira Wood Rail
Gough Island Moorhen