There are many web sites where you can find information about international sim cards, but I have yet to see a site where all are compared. Mostly, the sites like to promote a particular card (who sponsors them?)
These are the cards that I have found, in no particular order. I will add to this post as I find more.
- OneSimCard

- aether Mobile Global Roaming International Cell SIM Card

- Pangea Global Travel Prepaid SIM Card
- SmartFree SIM card
- oomobile Global Travel Sim
- Mobal World SIM

- ekit Global Premium Service
- ekit Passport Service

- ekit Passport Plus
- Go-SIM
- Global Traveller Sim
- Matrix International Travel Sim Card (from India)
- 0044 Global Sim Card
- SIM4travel
- National Geographic Talk Abroad SIM Card by Cellular Abroad

- GSM Travel SIM
- United Mobile (appears to be out of business?)
- Swiftnet TravelSIM
- OOMobile Travel Sim
I will try to take the time to produce a comparison table with whatever information I can find. In the mean time, I will continue adding to the list as I find more. If you have any ideas, please send them to me. [contact-form]