I couldn't really find a worldwide source for automatic registration, but
this report (pg.5) is a good start I think.
Primary method of registering voters:
Argentina - Local election officials add eligible citizens to the voter rolls from a national list of all citizens maintained by a federal agency
Australia - Election officials gather information from other government agencies to identify voting eligible individuals who are unregistered, and mail these individuals registration materials.
Austria - Local election officials automatically generate voter lists from larger population databases maintained at the municipal level. Inclusion on the municipal database is mandatory.
Belgium - Local election officials automatically generate voter lists from larger population databases maintained at the municipal level. Inclusion on the municipal database is mandatory.
Canada - Election officials use information collected from other government agencies to add eligible citizens to the rolls. If more information is required, election officials mail individuals pre-printed registration forms, with postage-paid return envelopes
France - Election officials automatically register 18 year olds using information compiled by the department of defense for military conscription purposes.
Germany - Local election officials automatically generate voter lists from larger population databases maintained at the municipal level. Inclusion on the municipal database is mandatory.
Great Britain - The government seeks out every eligible voter through an annual nationwide “canvass,” during which local officials mail or hand-deliver a form used for voter registration to each household. (my own comment on this: so this is not really automatic, but it makes registration very easy)
Sweden - The government automatically registers all eligible voters using information contained in the national population database.
And I'm sure there's a lot more out there.