Dignus' HLASM compatible assembler product for the IBM mainframe. It's available as a native assembler for OS/390 and z/OS, or as a cross-assembler hosted on Windows, AIX, Solaris, FreeBSD, Linux and Linux/390. http://www.dignus.com/
HLASM for MVS, VM, VSE, and the optional HLASM Toolkit Feature provide a comprehensive development environment for System/390 assembler applications. http://www-3.ibm.com/software/awdtools/hlasm/
The most comprehensive and flexible product available for the debugging of Assembler language programs which execute in the OS/390 (MVS) operating system environments on IBM (and compatible) mainframes. http://www.colesoft.com/
Now you can develop and maintain 370/390 assembler programs from your workstation! With the Tachyon Operating System you can test 370/390 Programs On Your Workstation! http://tachyonsoft.com/