Sjasm is a Z80 assembler originally developed by Sjoerd Mastijn. It is available to download, sources included, at XL2S Entertainment site.
This repository has been created from the original sources as a means to have them properly organized and easily browseable (and some cool features have been added to v0.39 along the way). It contains a Visual Studio solution holding a Visual C++ project with the sources themselves, and a C# project containing unit tests.
Two branches are available for the two versions of the program, they have been created from versions 0.39g6 and 0.42c. Switch to one of these branches to actually see code.
Download the latest binary of v0.39 for Windows from the releases section. Sorry, at this time there's no binary for Linux available (perhaps you can help?). The unmodified package for v0.42c is available there as well.
Apart from being included in their repsective packages, the documentation for v0.39 is in a text file together with the source and the manual for v0.42 is available as a set of HTML files in the repository, but you probably prefer to see it properly rendered.