Apple's RIA platform - Safari 3.1
Safari still has tiny, tiny penetration on Windows and higher on Mac (obviously) but the big players are still Firefox and IE. So can Safari bring HTML to the promised land? It's a tough call. But I think it's clear that this is Apple's RIA play, and that's a good thing. It's good for web standards because Apple is a strong company with a LOT of clout in the design/experience world, and it's good for companies like Microsoft and Adobe. Frankly, the standards process is broken and it's been incredibly difficult to innovate around HTML/CSS/JavaScript. Apple looks like they'r taking a lead and that will put pressure on Microsoft and Adobe to keep innovating.
So now I think it's safe to say we have three major RIA players: Apple, Adobe, and Microsoft. Mozilla also continues to innovate around Firefox with things like offline storage and Prism but I'm not sure they have a "rich" direction quite yet. Safari 3.1 may be a smallish blip on the radar overall, but it's a big, big shot in the battle for RIAs. If there's one absolute guarantee here, it's that the web is going to get a whole lot richer and that's always a good thing in my mind.
Update: I can't believe I forgot to mention this but j.m.galvin commented below that the majority of this work is in WebKit which is an open source project. Adobe uses WebKit in Adobe AIR and while I'm not sure what our plans are to incorporate the changes, the fact that this is all open source is a very good sign.