Ecco is a great tool available on many platforms including LINUX and Windows NT. Ecco allows you to compile Express schemas, easily hand make part 21 files, easily generate graphs of instances in part 21 files, and develop applications using the TCL/TK language (note I don't say easily on the last feature because I've not yet made this work).
Expresso is another great tool available for Windows NT. This tool allows you to read Expess Schemas and Interact with the data in part 21 files. It is also easy to write LISP routines for manipulating data in Expresso.
STEP Class Library are the oldest publicly available STEP tools. I've never succeeded in making these tools work.
WinSTEP is a Windows NT version of the fedex_plus tool in the STEP Class Library. This tool does successfully turn an Express schema into a set of C++ files which can then be used to develop applications. I have successfully generated C++ classes and HTML files using WinSTEP, but I've never succeeded in building an application using this tool.
GNUWIN32 by Cygnus is a GNU compiler that works on Windows NT. Conceptual Systems & Software have reported some success in using this compiler to build applications with the STEP Class Library, but so far I've been unable to duplicate thier success.