The purpose of the mod.smalltalk project is to integrate Smalltalk with the Apache Web Server via Fast CGI.

On the page describing Fast CGI, they say:

FastCGI is a fast, open, and secure Web server interface that solves the performance problems inherent in CGI, without introducing the overhead and complexity of proprietary APIs (Application Programming Interfaces).

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.

Updated Sept 7 2008: added seaBreeze link

Seaside

To get a good feel for Seaside development environment, read the excellent technical introduction by Shaffer Consulting.

Note that the article was written in 2005 and may be obsolete in parts. For example, Seaside now supports several object-oriented databases, including Gemstone.

March 19 2009: Pharo will be going beta with version 1.0 !!! I thought they said it would take three years ... glad it didn't :-).

Note: my understanding of these services is evolving and may be incorrect at this point. Updates coming soon probably ...

Of course, the best example of a Seaside-powered site may be the Seaside site itself. But there are several interesting ( and either cheap or free ) hosting sites for Seaside web applications.

Updated: Sept 10 2007

There is a small but persistent Smalltalk/OWL project. From their project information page:

The Smalltalk OWL Project was launched 1 Jan 06, the overview of the effort is:

A strong candidate for a Smalltalk RDF/OWL inference engine is the Smalltalk Open Unification Language (SOUL).

March 19 2009: Squeakland-EToys updated, see the EToys download page.
Updated July 10 2008: Pharo - new, leaner fork of Squeak. See related link.

From Wikipedia on Squeak Smalltalk:

Updated: Sept 10 2007

Smalltalk.org is the main link - it's a good kick-off point for Smalltalk adventures. The whysmalltalk.com site is also a good resource, even if some of the event announcements are several years old.

A great list of free Smalltalk books on the web.