Reuven Lerner, who writes the At the Forge column, talking about the different web technologies that have come up over the past 4 years and where are they going, writes (most interesting paragraphs. There's more in the column):
Another open source web server, AOLserver, has long contained an embedded Tcl interpreter. (...) Of these, I find AOLserver's technique of persistent, pooled connections to be the most elegant, since it works for all languages --although that is almost always going to be Tcl-- and scales extremely well. (...) Another application server taht has been getting a lot of publicity is the ArsDigita Content System, written and maintained by the ArsDigita consulting company and released under the GPL.(...) A volunteer effort known as OpenACS has been working to sove this problem (dependency on Oracle) by porting the ACS software to use PostgreSQL as a database. The software is not quite complete but does include a great deal of functionality and will undoubtedly improve over time.(...) We have already begun to use ACS for some large jobs, in no small part because of the very large number of working applications--not just underlying tools--that come with it. Moreover, the fact that ACS is free software and works with Linux makes it easy to work with since we can rely on the community to provide functionality, documentation, testing and bug fixes.(...)
There's also an article entitled "PHP4 and PostgreSQL: Building Serious Web Applications with Open Source Software" which praises the PostgreSQL improvements and robustness.