There's currently three plugins being developed for Unity that allw you to program in a visual way.
PlayMaker is basically a Finite State Machine editor. The only graphical representation is of the different states. The programming happens in Inspector-style panels which is a bit tedious but does have the advantage of removing syntax errors. It's a very well made package and available now from the Unity Asset Store.
Universe is more sophisticated. It aims to completely replace all scripting with an interface that seems deeply inspired by Virtools. But with a touch of Quest3D as it allows for realtime programming while the game is running! It's complex however and documentation is still lacking. Beta version is currently available from the Unity Asset Store.
uScript is a Kismet clone. It looks very clear and polished but I'm not sure how powerful it is. One thing I like is that it allows for dragging a Unity object into the graph. A beta version should soon be available.
I'm very happy with this evolution as I have been getting more and more frustrated with prototyping in code. Before I learned about these plugins, I had already decided to go back to using Quest3D for doing creative work. Now I may be able to stick with Unity.
If you have any experience with any of these tools, please share!