Looks like Adobe AIR
and Microsoft Silverlight
are getting competition: Appcelerator Titanium
has been released as public preview version. Basically it works similar like AIR: It's a native browser client running the web app. The difference to AIR: it's open source and your app can be distributed as single package, without the need of installing something like the AIR runtime before this. But I guess it will still need an installed flash player if you plan to include an .swf file in that package, unfortunately (unconfirmed yet), otherwise Titanium would be the perfect tool for distributing flash games as standalone versions on end user PCs, IMO.