• Link - VisualWorks Smalltalk

VisualWorks Smalltalk has drifted and evolved across the years eventually winding up with CINCOM. While not Open Source, it is a mature product and CINCOM has a liberal download policy for non-commercial use. They encourage you to register before downloading, but do not absolutely require you to do so, which shows the right spirit.

I've only installed it on Linux so far, but it is quite full and usable once properly configured. There seems to be a sort of friendly rivalry going on between the VisualWorks and Squeak communities.

From the Cincom site for their Smalltalk product line:

... applications are portable across many different platforms including Windows (95/98/ME/NT/2000/XP/Windows Server 2003/CE, Mac OS 9, Mac OS X, Linux (x86, PPC, SPARC), AIX®, SGI® IRIX®, HP-UX and Solaris™, delivering a true write-once, deploy-anywhere environment. In addition, Cincom Smalltalk offers full support for all the relevant communication and interoperability standards, such as XML, SOAP, WSDL and Web Services.

VisualWorks also has good distributed capabilities and support for Seaside web server, SIXX, etc.

Cincom Smalltalk Wiki. 

Cincom's Public Repository for user projects.