On a Mac, you can type Romaji and have the Mac help select Kanji. (Using Pinyin for Traditional Chinese characters works the same way on Macs.) Open "TextEdit" (in the Applications folder). Then setup the Mac as follows:
Open System Preferences, select International, click "Input Menu" and in the items there, click the box to select a Japanese input method - on Mac OS 10.4 Tiger this would be "Kotoeri." Also click the box that says "Show input menu in menu bar."
You will now have a US flag on your menu bar at the top right of the screen. By clicking it you can switch the input menu to characters of a different language, including Japanese. In this case, click the flag and select "Hiragana."
In your Text Edit document, type the Romaji and press the space bar. If the kanji is not correct, press the space bar. You will then see different kanji choices. (Pressing the space bar is a way to select the appropriate kanji.)
Traditional Chinese is also available via the same method; you would check the boxes for "Traditional Chinese" in addition to Kotoeri. Chinese (or Japanese) characters then become available for use in email as well as TextEdit. You can use Pinyin to give you the characters.