The developers of Wunderlist chose to utilize the Titanium framework for its creation, which provides a powerful toolset for building cross-platform apps ().
Titanium's capabilities extend to developing applications that can seamlessly run on Windows, Mac, mobile devices, and beyond.
One notable feature of Titanium is its inclusion of Mac-like UI controls, giving apps created with it a familiar aesthetic.
Edit
To learn more about Titanium Desktop development, visit: http://www.appcelerator.com/products/titanium-desktop-application-development/
Thanks to Titanium, creating apps that match the visual style of different operating systems is made easier—Vista apps look like Vista apps, XP apps resemble XP apps, and Mac OS X apps appear fully native.