Web OS apps are debugged using nothing more than your computer’s web browser. The MOJO SDK sets up shop as a local server on your computer, processing your newly-created web-apps for viewing in your web browser of choice.
In-browser application emulation bodes well for web-developers hoping for an easy and and familiar development environment. Developing an application (at least in the low-end of the performance/functionality spectrum) for the Web OS is apparently as simple as making a web-app for the iPhone.
Hit up BGR for the rest of the MOJO SDK gallery.