Robert Nyman and Rob Hawkes touched on this, too, but I’ll add that one of the signifcant parts of Opera’s announcement was that Opera adopted Chromium. This means the WebGL, Canvas, HTML5 forms, 2D graphics implementations–all that stuff will be the same on Chrome and Opera now. Same APIs, and same backend implementation. Since Opera is Chromium-based, you can feel confident that your cutting-edge work will be compatible with Chrome and Opera simultaneously.

I should also point out that all Opera browsers will be adopting Chromium. So Opera for Windows, Mac and Linux and Opera Mobile (the fully fledged mobile browser). Even Opera Mini, the thin client, will be swapping out the current server-side rendering farm based on Presto with one based on Chromium.

.. and the WebKit Nightly. What is that?

It’s the mac port of WebKit, running inside of the same binary that Safari uses (though with a few underlying libraries swapped out). So its behavior and feature set is congruent with what you’ll find in Safari. If you want to go back to middle-school analogies, think of it as… WebKit Nightly is to Safari what Chromium is to Chrome.