If you are familiar with the embedded browser in the Lotus products you may or may not have heard of BrowserFunction, most likely not! The problem is this class is not very well documented and was part of E4 and is scheduled for Eclipse 3.6. Notes 8.5.2 does however support this SWT class so you can begin to play with it once 8.5.2 is available.
The Attachment viewer is such an application, you can check out the code on OpenNTF. I don’t use the BrowserFunction in the OpenNTF version but I have already begun to make an Lotus Notes 8.5.2 version of the plugin that has much better integration with the Notes client. If you are writing side bar plugins and want to standardize on writing in a single technology you should check out the SWT Browser and the BrowserFunction support.