Erlang can be installed from http://www.erlang.org. There are versions for Mac, Linux, and Windows. In addition, most popular Linux distributions have packages for Erlang. As of this writing, the current version is R15B01.

Many modern Linux distributions will feature Erlang and Yaws in their basic package system. So from Ubuntu, doing a simple sudo apt-get install yaws will install Erlang and Yaws (and any dependencies). Similarly, Fedora Linux includes both Erlang and Yaws as part of the basic packages, which can be installed via yum.

Yaws can be found at http://yaws.hyber.org and downloaded from there. Once again, most Linux distributions will include packages. As of January 2012, the current version is 1.92.