FontBoard
Freely downloadable keyboards for Arabic, Cyrillic, Esperanto, Hebrew, Maltese and Yiddish. They can be used in Windows 2000/XP as well as in Windows 95/98/ME. The site is maintained by Gyula Zsigri.
http://zsigri.tripod.com/fontboard/

Tavultesoft Keyman
Tavultesoft Keyman allows you to create your own input methods with a visual interface or a powerful programming language for just about any language in the world. It works with nearly all Windows-based applications and best of all it is free for personnal use.
http://www.tavultesoft.com/keyman/

Uniqoder [This link generated an error during the last automatic check]
UNIQODER is a freeware add-on to Microsoft Word for Windows 97-2000 which enables you to enter Unicode characters quickly and easily. It focuses on Extended Latin, phonetic and Greek characters, including special routines for diacritics.
http://hem.fyristorg.com/dahloe/uniqoder/

VietIME: Cross-platform Vietnamese Input Method Editor
A Java-based Vietnamese input method editor (IME). Enable input of Vietnamese Unicode text in Java's AWT (TextArea and TextField) and Swing text components.
http://vietime.sourceforge.net