Note, this information here is provided "as-is", and no warrenty as to it's accuracy. The mobile world changes very quickly, and while I'll keep this post up to date as best I can, check out some of the links before making a decision. You make your own choices.

I found this Wikia link in c't magazine (19/10), which lists quite a few countries, but not always in the details we have below.

Europe

Ireland - All networks
Germany - VodaFone
Germany - Tchibo Mobil (MVNO)
Germany - Aldi Talk (MVNO)
UK - Networks
UK - Asda (MVNO)
UK - Tesco (MVNO)
Sweden - 3/Tre
Spain - Vodafone
Switzerland - Swisscom

North America
USA
Canada

Africa
South Africa

Middle East
UAE - DU

Asia/Australia
Australia
Hong Kong
Malaysia - Digi
New Zealand - 2degrees
Thailand - AIS 1-2-1 also More notes on AIS