BibDesk also supports Mac OS X Services to add references to an existing bibliography. Services are found under the application menu next to the Apple menu, and operate on selected text.
To use this feature, select BibTeX or RIS text in an application that supports Services (such as OmniWeb), and choose the "Add to Bibliography" menu item of BibDesk's "Services" submenu. The selected text will be added to the frontmost document in BibDesk automatically.