Obviously, there are different chapters for these organizations. So direct donations should go to the Japan chapters of the Red Cross (NOT the American or wherever else Red Cross) and Save The Children.
I don't think Tzu Chi is in Japan since it's a Taiwanese thing, but I'm fairly certain any of their pages is okay.
Or, if you want to be sure about it going to Japan efforts, even if it's through the UCSD JSA, there's this page. Like I mentioned before, they're in contact with the governor of the Miyagi prefecture, so they're trying to send the money directly to them.
[And now if you will excuse me, I need to go write a post about a terribad UCLA student-written article about that "Asians in the Library" video controversy, in which they make uninformed judgments about UCSD. Those jerks.]