WinCompose - WinCompose supports the standard Compose file format.
PopChar - The character map that works!
AllChars - If you're new to AllChars: start with the Introduction, see the FAQ and Manual. And finally download it. AllChars is now Open Source, usage is free and the sourcecode is available.
BabelMap - Unicode Character Map for Windows
Holdkey - Holdkey is a Windows program that makes it easy to type characters with accents..
Gucharmap - Open source character map for the GNOME desktop environment.