Import and Export (3)

Bagpipe Music Writer: *.bww
Band-in-a-Box: *.mgu *.sgu
Capella 2000 (3.0)+: *.cap *.capx
Guitar Pro: *.gtp/gp{3,4,5}/gpx
Overture: *.ove
ABC: *.abc  (via plugin, using a webservice)