From this page you can download various parts of the latest version of Yacas, version 1.2.2.
The source code is offered in a .tar.gz file, with all the source to generate everything relating to Yacas.
The latest version of Yacas is yacas-1.2.2.tar.gz.
The online documentation in html format can also be downloaded as one tar.gz file, manual.html.tar.gz.
In addition, there is the postscript version, manual.ps.tar.gz, and the pdf version manual.pdf.tar.gz of the manual.
Older versions, and versions for other platforms, like Windows, can be found here.