The latest in blatant Linux-software-ripoff-jobs is Kivio, a Visio clone.

First of all, why would you clone Visio? Even more than Outlook sucks and thus Evolution-as-an-Outlook-clone sucks more, Visio has an even more horrible UI, IMO.

If you want a great diagramming tool, check out GlyphiX... I hope they port it to Mac OS X (it's currently available for Mac OS X Server and NT/2000 through the Yellow Box libraries that (only) come with WebObjects).