I've been spending a bit of time playing around with the Yahoo! products available through Yahoo!Xtra.
I generally use this at home on my mac and was kind of disappointed that Pro Mail was not officially compatible with either Safari or Firefox. It seems there have been some upgrades recently which means that this compatibility now exists - works much faster on both browsers making it a much more useful service.
I was talking to the team at Telecom about the roadmap for more improvements - can't say anything too specific about them, but from the screenshots I saw, the evolution of this product is simply stunning. Expect to see more this year with a more social networking orientation and a better calendar product as well. The speed they are moving at is phenomenal. They are embracing OpenId very enthusiastically so expect to see more of an ecosystem approach from the team at Y!. Interesting observation from our team - our use of webmail in NZ is really low compared to the US - wonder why that is?
There is a blog from their developers if you want to know more about what they are doing with the mail product.
The other piece of capability I have been playing with is Yahoo! pipes.
I found this while trying to get comments from Plaxo Pulse integrated into my blog (more on that later - it isn't quite working yet). Its a great piece of capability - allows me to build, manipulate and output feeds by dragging modules around, applying filters and connecting them up. Brilliant - even I can (kinda) make it work.
There are some real benefits to this relationship in terms of open-ness that we didn't see with our previous partner.