This is touted to be Opera's answer to Firefox's extensions. Widgets in the literal sense are small programs that run on the desktop, or the application which supports them. Its not clear at the moment if these widgets will run inside the Opera application, or run on the desktop and use Opera's engine. But good times are on the way for the Opera community (and I can see a very good reason for me to go back!). Original article here.
When one hovers over a tab, they'll get to see a thumbnail preview of the contents of that tab. This feature is there in the latest beta of IE7, but Firefox's engine prevents something like this from being added. The closest one can come to this is by using certain extensions like viamatic foxpose. But Opera will have this without any addons. Original article here.
I see good times for Opera lovers! And widgets will certainly get people buzzing with excitement at the prospect of getting to code for Opera. :D