TUAW has posted a summary of developers & apps to help Japan. In short, simply download or purchase some development tools or iOS apps, the developer will donate money to Japan via various means.
Please note that some effort is time limited. For instance, Capcom will give 100% of their iOS app sales until 21st March, 2011.
If you prefer, you can help directly by donating to your local charity, such as Red Cross.
Japan Red Cross
American Red Cross
Hong Kong Red Cross
Let’s help Japan to relief the recent earthquake & tsunami crisis!