PopChar - The character map that works!
WinCompose - WinCompose supports the standard Compose file format.
Free Virtual Keyboard - Free Virtual Keyboard is a free, lightweight, multilingual and finger friendly virtual keyboard.
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.
Hot Virtual Keyboard - A replacement for Windows' built in virtual touch keyboard.
Gucharmap - Open source character map for the GNOME desktop environment.