It's been discussed, and now they're starting: last week three different virtual worlds platforms came to my attention. Their common point? They use webgl, wich means that they can be used in chrome/firefox/safari whitout requiring additonal plugins.
Sirikata is still experimental, looks good in screenshots, requires a download. I haven't tried it yet... edit: tested it, requires a very buggy and unstable client, and quite unusable at this time.
Umniverse looks very clunky and not very well developed graphically.
The most promising one, as far as i know, is Cloud Party. They use some very ingenious ideas. Login is facebook based, easing the access process, and integrates as a facebook app (which is where i found about it). Hosting is cloud based in amazon servers. Has a second-life look and feel and allows customization of virtual spaces and scripting. For regular users, so far it allows creating meshes or importing 3D objects (with an annoying 100 object limit, wich might seem much but... a vrml model built with multiple primitives/ifs reaches that number quite easily) into a virtual house. For me, the mesh import option is the most interesting one in this very beta world. So far, the build menu in Cloud Party allows .obj meshes and jpg/png textures import. I wrote a couple of quick overviews about avatars (pretty basic, so far, and for those of us used to avatar studio's customization) and some general impressions.
The virtual world as facebook app is something pioneered by Vivaty. May Cloud Party have better luck (they're also web based and working on a mobile/tablet access solution) and... include more import options. It would be great if a vrml file with objects, interaction, etc could be imported or converted to webGL as a personal space in this world...