What are some alternatives?
When comparing Extra Keys and Holdkey, you can also consider the following products
Event Viewer - Get help, support, and tutorials for Windows products—Windows 10, Windows 8.1, Windows 7, and Windows 10 Mobile.
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.
Gucharmap - Open source character map for the GNOME desktop environment.
WinCompose - WinCompose supports the standard Compose file format.
SymbSearch - A tool for searching unicode-symbols and copying it to the clipboard.