The following sites are listed alphabetically, starting from the first letter after www. This list is not complete and will be regularly updated.
http://archive.folx.org | A large collection of sheet music, videos and audio recordings. Users can sign up to the site and add their own contributions.
http://www.diato.org | The site of Bernard Loffet, an instrument maker and a diatonic accordion player. It includes a number of tabs of all music styles and for all levels. Very very useful!
http://www.diatofiddle.com | Jean-Marc Siche’s website, a great repertoire of tablatures for diatonic accordion and fiddle, classed by geographical area and genre.
http://www.diatonia.net | A large repertoire of tabs, meticulously classed by genre and instrument type.
http://www.erwan-diato.fr | Erwan Tanguy’s website. One of the best sites dedicated to diatonic accordion, with lots of tablatures and very clear tutorials and exercises to improve playing technique.
http://www.organetto.it | An Italian portal about diatonic accordion: tuition, music, sheet music and tabs, organology, ads for used instruments.
http://www.tabledit.com | The site of Tabledit, a gorgeous tablature software.