Open Source RIA Framework For Mac: Fluidium
Fluidium, similar to Adobe AIR but open source, is a platform for developing Rich Internet Applications that run as native Mac OS X desktop apps.
Although there are many new applications that are built as web apps., native Cocoa applications still provide the best user experience on Mac OS X.
Fluidium can wrap your existing web app in a pure, native Mac application shell to provide the best of both worlds to your demanding user base.
It is based on the same open source technology that powers the popular Mac OS X Site Specific Browser: Fluid (which was discussed here).