VisualWorks Cincom
VisualWorks is the direct successor of Smalltalk-80. The abstraction regarding the platform is the most distinctive feature of VisualWorks. Platform peculiarities are an integral part of the virtual machine. This allows an image to be executed on all supported platforms without any alterations. These platforms are Windows, Macintosh, Linux and many other UNIX systems. VisualWorks is available in a non-commercial version with similar conditions to ObjectStudio.
ObjectStudio Cincom
ObjectStudio is a Smalltalk development system which is excusively available for Windows. It has a port to each standard databank on private and native drivers as well as legacy systems (such as CICS, MQ Series, APPC and EHLLAPI) which makes it ideal for connecting existing mainframe systems to a modern user interface. ObjectStudio is available in a non-commercial version without time or feature limitations.
Squeak was developed as a free Smalltalk-80 implementation by a group surrounding Alan Kay, who is one of the fathers of Smalltalk and worked at Apple and later at Disney. The virtual machine itself is realised in Squeak and thus easy to maintain and to port onto new platforms. New computer operating concepts are being developed in Squeak. This applies particularly to the user's guide and the interaction with multimedia data. Squeak has an active community of users who push its development, similarly to Linux.
Smalltalk/X eXept Software AG
Smalltalk/X is a Smalltalk system which has the special feature of containing a translation from Smalltalk to C, with which real DLLs or "shared libraries" can be created. Another special feature is the insertion of the inline-C code directly into the method source code for the linkage of external C/C++ procedures or to tune time-critical code. Smalltalk/X is available for Windows and most of the UNIX systems. It is also available commercially with no license fees.
Initially, GemStone/S was developed by GemStone Systems and later it was sold to GEMTALK SYSTEMS who still distribute and develop it further. GemStone/S is a Smalltalk-based development platform that provides a highly scalable, high performance database server, which supports various Smalltalk systems as well as Java as clients. GemStone/S uses the same object models as Smalltalk, which ensures a seamless integration in Smalltalk clients. Objects and classes can be moved easily between client and server. GemStone/S is available as a free version without a time limit for Linux with certain functional restrictions.
GNU Smalltalk
GNU Smalltalk is a free, non-commercial implementation of Smalltalk-80. The first version was already available in the mid-80s and is currently under further development. Although GNU Smalltalk has a graphical user interface, the main application area as well as the direction of development are, in fact, server programs and scripts. GNU Smalltalk resembles the more classical script languages such as Perl or PHP. GNU Smalltalk runs on all UNIX systems with POSIX-compatible libraries.
VA Smalltalk Instantiations
VisualAge was probably one of the most popular Smalltalk systems, developed and distributed by IBM on the basis of the cross-language development platform VisualAge. In 2005, on-going development was handed over to the company Instantiations, which distributed it under the name of VA Smalltalk. VA Smalltalk provides a lot of various components, for instance interfaces for databases, various transaction processing monitors other IBM server services and can be integrated into a Java environment via RMI. VA Smalltalk is still fully compatible with VisualAge for Smalltalk and supports the latest versions of the operating systems Windows, Linux, Solaris and AIX.
Pharo Smalltalk
Pharo is a free open source project for a clean and innovative Smalltalk environment. It is meant to achieve an attractive platform that creates "mission critical" Smalltalk applications based on a stable, small core system and excellent development tools.
Little Smalltalk
Little Smalltalk is a small experimental Smalltalk system aimed at writing the most minimalistic Smalltalk applications. The VM consists of only around 1800 lines of source code. Little Smalltalk is a minimalistic system that is easy to understand and extend. Little Smalltalk is free for non-commercial use. 
Smalltalk/JVM Mission Software
Smalltalk/JVM seamlessly integrates Smalltalk with Java. It compiles Smalltalk classes in Java class files which can be loaded into every Java virtual machine. Smalltalk and Java classes thus exist in a parallel manner within the same virtual machine and communicate with each other. Smalltalk/JVM provides only a few of its own components because it has full access to the SDKs, available with Java, and these can therefore be used on seperate applications. A free, non-commercial Windows version can be downloaded from the web address. Additionally, SUN JDK 1.3 and the Sun Java Servlet Development Kit are required.
Dolphin Smalltalk Object Arts
Dolphin Smalltalk is one of the newest Smalltalk systems and is exclusively available for Windows. Interfaces for all common Windows components such as ODBC, OLE, COM, ActiveX, DirectX, and so on are integrated into Dolphin Smalltalk. The user's guide for Dolphin Smalltalk is based on the common and known development systems in Windows in order to ease the transition for the developer. Dolphin Smalltalk is available in a trial version with time limitations.
Smalltalk MT Object Connect
Smalltalk MT demonstrates the most consistent integration into the Windows platform and is also only available for Windows. With Smalltalk MT, native Windows applications as well as DLLs and ActiveX Controls can be created. With direct integration and the usage of Windows functionalities, programs can be created with low resource requirements. Smalltalk MT is available in a trial version with time limitations and can be downloaded from the Object Connect online server.
PocketSmalltalk is a free Smalltalk development environment that translates Smalltalk applications into the binary format of the Palm OS. The generated applications are real Palm OS applications that run well without a virtual machine and therefore consume less storage space. PocketSmalltalk exists in two versions. The most updated version is currently based on Dolphin Smalltalk and is only available on Windows platforms. The other version is realized in Squeak and is available on a number of platforms.
Smallscript sets itself apart from the other Smalltalk systems through its distinctive script character. Smallscript allows the creation of applications with graphical interfaces, however the main application areas are server applications and Smalltalk scripts. Smallscript is no longer in development.
LSW Vision-Smalltalk
LSWVST is a Smalltalk with deeper integration with operating systems as well as .NET connectivity.
LSWVST is a proprietary Smalltalk which is used to develop applications in LSW.
ambraiSmalltalk Discussions:
Ambrai Smalltalk is an application and development environment of the Smalltalk Language for Mac OS X which hasn't been updated since 2007.
SharpSmalltalk and #Smalltalk    
SharpSmalltalk runs Smalltalk under .Net framework.
Smalltalk for .NET and JAVA.
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Smalltalk Development Systems