I've seen it on many Boeings especially.

Probably to aid in braking on poor surfaces

to help with the breaking - l guess because they are test aircraft, they are not too sure re breaking distances and speeds. Loads of military aircraft have them due to the short runways they land on

They don't. The drag created would negate any test flight results and also make the aircraft extremely unstable. High speed aircraft use a parachute for braking.

These are generally stall/spin suppressors.

Also, there is a line of chutes for small planes called 'second chance' or similar.

It is called an anti-spin 'chute. Test aircraft are deliberately put into attitudes and flight conditions they should never meet in service, which may result in a spin, a condition from which it can be extremely difficult to recover, especially until the right technique has been established. If need be, the 'chute acts to pull them out of the spin and allow the pilots to recover to normal flight.

It is called a drogue or trailing cone. It is used for calibrating aircraft static pressure systems. It is trailed behind the plane far enough so it is not effected by the disturbed airflow from the plane.

http://airliners.net/photo/Boeing-737-8EH/1021515/L/&width=1600&height=1079&sok=&sort=&photo_nr=&prev_id=&next_id=

Read the comments below the picture.

and it is far soo small to aid in braking & even if it was, they would not deploy it until the wheels were on the ground.