Extra Keys is described as 'software input panel for typing international accented and other special characters'. There are more than 10 alternatives to Extra Keys for a variety of platforms, including Windows, Mac, iPhone, BSD and PortableApps.com. The best alternative is BabelMap, which is free. Other great apps like Extra Keys are Character Map (Free), WinCompose (Free, Open Source), Rocket (Freemium) and Free Virtual Keyboard (Free).
BabelMap is a free character map application for Windows that allows you to browse through the entire Unicode character repertoire of over 107,000 characters, or search for a particular character by name or by code point.
The character map that works! PopChar is an application that lets you insert special characters, umlauts and foreign letters into any document. With this character map, there is no need to search for or remember complex keystrokes.
Uosk is a light tool that opens any ASCII, ANSI, UTF-8 and UTF-16 text file and converts words (e.g. separated by spaces) into buttons. Clicking one button the text snippet is pasted into any text editor (Notepad, WordPad, MS Office, OpenOffice...).