ACS is dead since about 2001. The latest incarnation of ACS is OpenACS, available at http://openacs.org
-- EGLim
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A free and open-source framework for building web services. In prior lives, it was written using Tcl and AOLserver. Now, it's in Java. http://developer.arsdigita.com.

It does all sorts of things for you: it has a persistence layer for database independence and to isolate code from changes in schemas; user and session management; a bunch of services such as workflow and search that can be used by applications that use the ACS.
There's also a content management system. 
-- RichardLi

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Isn't this just another word for ApplicationServer?
The reason I ask is because PhilipGreenspun traditionally had a big aversion to the concept of application server at all (notably slamming Kiva).
-- WillSargent


It depends on your definition of application server. Some people view application servers as the environment that runs in the middle of your n-tier architecture e.g., servlet/EJB containers such as WebLogic, WebSphere, Oracle iAS. ACS is a system that runs on top of this layer (in the same way that AtgDynamo, Turbine, WebLogic commerce server, or Oracle portals runs on top of the servlet/EJB containers). 
-- RichardLi

(Note: switched AtgDynamo to second category above -- WillSargent) 
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So you would classify ACS as an application rather than an ApplicationServer.  This makes sense to me, except that I was under the impression ACS came with its own infrastructure (connection pooling, etc).  

-- WillSargent

I would better name it a application framework. Application servers are hybrids of application framework and application platform.  --ManfredSchaefer
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ACS does provide a lot of infrastructure, but in a database-independent and application-server-independent way. For instance, it includes a persistence layer that handles mapping objects to relational databases. IIRC, the ACS connection pooling mechanism is a thin wrapper around JDBC so that it integrates with Log4J (Apache project open-source logging system). --RichardLi
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It has a pretty cool memoization function that lets you wrap any function call in a memoize function, specifying how long you want to cache the value for. Using it is really quite painless, almost making up for the fact that it's in Tcl.

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The ACS "Intranet" module (the part of ACS used by ArsDigita itself to manage its projects) has been reincarnated and is not part of Project/Open at http://www-project-open.org/.