Smalltalk/X
Smalltalk/X is a free implementation of the Smalltalk programming language.
For download, see http://www.smalltalk-x.de.
The Smalltalk/X package includes IDE, class libraries, an OS independent window framework, debugger, compilers, UI builders and many other tools.
Smalltalk/X is a "state of the art" system, with a modern generational garbage collector, incremental non-disruptive background collector for old objects.
It includes a big class library with packages for XML, SOAP, database access (also NoSQL), communication protocols, web clients and servers and much more. It also includes a modern IDE which supports syntax highlighting, refactoring, code instrumentation, coverage and unit tests.
In addition to bytecode execution (which is translated just in time to machine code), it also includes a Smalltalk-to-C compiler which is used to generate binary executables and binary class libraries without a need for the bytecode engine and without the IDE (if packaged as such). Both UI and non-UI applications can be generated (eg. web servers, Windows services etc.)
Although not normally used as such, it can be executed as a scripting language with a REPL loop.
It is available for Windows, Linux, OS X and other Unix-based platforms (upon request).