This may seem as wierd question, but between the head tube and down tube on my atb, which has an Alloy frame, there is an extra piece sort of welded onto the bottom of the down tube, right next to the butt (to the head tube), about an inch long.
Ok, so why is that added to the frame, is it to strengthen it ( my guess), or is there some other reason?

You are right, it`s to srengthen the head and down tube junction.

Steve.!

What is that thing?
 

It is called a "gusset". And Steve is correct it does add strength to the area that is joined. I first saw them back in the late 70's and early 80's on BMX bikes. They made a isurgence to MTB, I first noticed them on slalom rigs. Great idea for a tig welded joint.

cheers,
just one of those things that bug you if you don't know!
Edward