Inner Frame Mounted Carrying Bag
There are many available.  Here is one example: http://www.ebikestop.com/axiom_cascade_frame_pack_black-BG6638.php

Attaches to the seat tube and the top tube, providing a triangular storage space.  This one also is padded to aid with portaging (if you have a big enough triangle).  

I know some people who tour with home-made frame bags that take up the entire triangle.

These bags are a great idea because they keep your load close to your center of gravity.