Alternatives To Applets On The Client Side?

Choron asks: "Anybody will agree that Java Appplets are great for developing complex applications for the client side, with good (but not perfect) portability. Now apart from Tcl/Tk or esoteric technology such as embedded Inferno or developing a plugin (so much for the portability), are there other solutions for developing both portable and advanced GUI applications embedded in a browser?" Currently, I think Java applets are the only way to do client-side applets with any degree of portability, but who is to say that this will always be the case. What may the future hold for applets, and will Java be the sole driving force behind the technology?