The desktop version. Pricing is excellent - free
Their desktop solution is quite interesting. Since I'm using a laptop, I'm often disconnected from the network. But I can delegate my co-worker who has a desktop PC constantly connected to the network to sync with my phone. It is working quite well so far.
I am not sure about pricing for the corporate version. I think it costs something like $1999 per year. My company has already bought 5 Blackberry BES licenses for my group. So I dare not mention to them getting the corporate MyMail software.