A+ is a programming language developed at Morgan Stanley and released open source in January 2001.
The main source for all that is A+ is the A+ home page.
I have packaged A+ for the Debian GNU/Linux distribution, and am the official Debian maintainer for the packages (neil@debian.org). They have been part of the official distribution since the sarge version (3.1). The packages are available from your favorite Debian mirror. You can download and install them with dpkg, or use dselect, apt-get or aptitude.
To just run an A+ program, all you need is the aplus-fsf package. To develop A+ programs in the XEmacs environment, you should get all of the packages. The easiest way to do that is install aplus-fsf-dev and xfonts-kapl. The former is a meta package that does nothing but depends on all the others except the latter. The latter must be done separately, because Debian policy forbids packages from depending on X fonts packages.