Felt markers are generally very useful, until you need to create a mix of thick and thin lines (try switching from flip chart to writing on a postit note using a standard Sharpie).
Today’s invention is a felt marker with a ‘nib’ on either end and sharing the same ink reservoir.
Each nib would have its own sealable cap as usual, but one would be much finer than the other.
I just spotted these in an art shop
http://www.werd.com/11312/pantone-universe-twin-marker-sets/
not sure if they were created before or after this post.