JGraph Ltd. has released mxGraph, a cross-browser javascript-based diagramming library (so, no Flash is required). The demo looks impressive, though the cross-browser part does not include Opera (and don’t draw anything you actually need, because you won’t be able to save or export).

Another sticking point is the licensing. You need to contact JGraph to obtain a copy of the Javascript source (simply downloading the files used by the online demo does not give you a license). While non-commercial licenses are free, a commercial license will run you into the thousands of dollars, depending on the number of end-users.

As far as technology, a quick look in mxClient.js reveals that mxGraph seems to use either SVG or (for Internet Explorer) VML:

isBrowserSupported:function(){
return mxClient.IS_VML||mxClient.IS_SVG;
}

This begs the question of why mxGraph does not work with Opera, which supports SVG just fine.

Overall, I think that, while mxGraph might be a fine bit of work, it suffers in terms of cross-browser support and licensing restrictions. You may want to hold your breath before basing your project on this library. It’s possible that something more convenient will come out (or that mxGraph itself will address its shortcomings).