Just Spent the past couple of days learning about Adobe Air and Adobe Flex
I've shamelessly ripped someone's UI from their program, but It's all for a good cause... personal fitness!
AIR Lets you run Flash or HTML built stuff like a Desktop Application and runs the same on Mac and Windows and soon Linux
Flex lets you build complex applications with lots of UI with ease.
The only problem with this as always is the fact that Flash performs like shit on PPC based Macs, I can't speak for Intel macs as I don't have one, unless you count a mini, which is underpowered anyway.