The official site, with info about all productions: Mamma Mia!
Info about the London production: The Musical Mamma Mia!
Mamma Mia in Germany: Das ABBA Musical Mamma Mia in Hamburg
The official Dutch site: Mamma Mia!: de Musical
The German fan club site: Der offizielle Mamma Mia fanclub
Part of a larger Abba site: Robert’s Mamma Mia! page
A fan site with some fun stuff! Mamma Mia! The Musical