Interaction has up a good page listing organizations that are accepting donations for Japan relief. Of course, contribute to your favorite charity first, but if you’re new to charitable giving, it’s a great start page listing charities that do good works well.
The American Nuclear Society has set up a page to specifically benefit nuclear plant workers and their families. If you want your donation to be thus directed, visit their page. ANS provides a link there so you can ask about its fund before donating – always a good idea so you can be comfortable with how much of - and how - your money will be directed.