Top 10 iCal clients & interoperability Tests

Hi, I want to know if anyone has done any marketing research on the top 10 
most popular iCal clients in the market?  I want to see if there is such a 
thing.

I know that MSN supports iCal protocol as well.  Do you know if other public 
providers are supporting that?

Do you know where I can get information or a consortium for doing an iCal 
RFC2445 interoperability Tests?

Thanks.

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