You could say that widgets (or gadgets, as Google likes to call them) are small applications that bring together a lot of information relevant to you. Now what if these gadgets would communicate with each other by sending small messages? PubSub is a new beta feature available at iGoogle. You won't find too many gadgets that use this feature, at least for now.
"PubSub allows multiple gadgets on the same page to send and receive data from each other. In other words, you can now build a gadget that communicates back and forth with one another. This introduces a brand new concept and strategy involved when writing gadgets. Information is no longer constrained to fit inside a single gadget. Instead, you can now split up various pieces of information amongst multiple gadgets and allow them to communicate with each other to paint a bigger picture. Gadgets now have the ability to be more closely integrated with one another and present a network of information to users."
This works if you add at least two gadgets, so it makes sense to create an entire tab with interactive gadgets (here's a sample tab). For example, you could have a gadget that includes a search box and other gadgets that show search results from different sources. Or another gadget could collect events (new email, new event, breaking news) and cleverly organize them based on your preferences.
"PubSub is a new framework which allows 'publisher' gadgets on iGoogle to communicate changes to 'subscriber' gadgets that have declared interest in those changes. This is currently available only on iGoogle and publisher/subscriber gadgets must be on the same page."
If you intend to write gadgets that use this new feature, read the documentation. How would you this framework?