Adobe Flash Builder (or the Eclipse IDE with the Flash Builder plugin installed - ships as same product sku) and target building Adobe AIR applications.
You'll then be able to deliver very Mac-like applications that run on both Windows and Macintosh (there's also an AIR runtime for Linux too). With this particular IDE choice you can choose to make either Windows or Mac OS X your primary development platform.
The resulting application projects can be made open source projects because Maven Flex-mojo plugin can be used to build Flex and AIR applications (and ant as well). So one doesn't need the Adobe Flash Builder plugin (a commercial item) to build the project. Plus the Flex and AIR SDKs are free to download.
The Flash Builder provides for the visual design tool, source level debugging, advanced datagrid, and charting and automation classes. It's a nice-to-have if you're a primary developer to the project doing a lot of the visual GUI design.
In our shop we also use Java and Tomcat on the server side with open source Java libraries, such as BlazeDS, Spring Framework, and iBATIS. Our software solution stack is open source across tiers, top to bottom. We deploy Tomcat on either Windows or Linux servers. When we need a message broker, we use open source ActiveMQ and integrate it to BlazeDS. That way our Flex/AIR clients can be subscribers to JMS topics.