Brass is a feature-rich general-purpose Z80 assembler.
Click here to download the latest Brass executable. Currently at version 1.0.5.3
Brass requires an implementation of the .NET 2.0 Framework. Microsoft's works best for Windows, for other operating systems (such as Linux or MacOSX) Mono is up to the job.
You can download the Latenite XML version of this manual here. Currently at version 1.0.3.2