WPF/E allows you to use a subset of XAML on multiple platforms (Windows and Macintosh so far but there are rumors of "possibly" even Linux and Solaris). Microsoft has released the first public CTP which you can download here.
Once you install, there is a collection of WPF/E Samples on Channel9 including this Sprawl Game which (what can I say) feels a lot like a Flash game!
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