I recently began work on a new music notation editor called ScoreChaser. A
prototype release can be found at
sourceforge.net/projects/scorechaser.
Its intent is to be a program in the vein of Rosegarden (and indeed, it
borrows quite heavily from RG, at least in these early versions), but with
groupable tracks, tablature support, and (eventually) multiple simultaneous
time signatures.
It's currently just an initial release with very limited functionality, but it
does demonstrate the groupable tracks and the beginnings of tablature
support.