Free Windows keyboard driver that extends AltGr to provide comprehensive accented character support (including multi-accented characters), international punctuation, IPA, medieval characters, mathematical and non-mathematical symbols, and more.
Designed for UK keyboards and installs as a regular keyboard layout option. I might adapt it for US keyboards if there's enough interest.
The driver should be stable (built using MSKLC, which is mature) but classified as beta because it has not yet been tested on a wide variety of systems (please give feedback!)
Note that this can't display characters not supported in fonts on your system! Also if you enter an unusual symbol in some programs such as MS Word the font might switch to one which includes that symbol – this isn't anything the driver can control.