Most enterprise development environments already have pre-selected infrastructure components. Regardless, I will select my default preferences here for those who want to simulate an enterprise environment on their own.

• Operating System
  Linux will be the obvious choice here. I myself will be using Windows 2000 server primarily because I can't bother with a separate O/S installation.
• Database
  Many enterprises will not be able to move away from their commercial vendors like Oracle and IBM. This is exactly why any decent reference architecture would be largely independent of the specific database vendor.
  If you don't have the database environment setup yet, feel free to use the set of tools that I'm using:
  MySQL - Download and install the MySQL database, MySQL Control Center and MySQL Administrator. You will also need the MySQL Connector/J JDBC driver.

• Application Server
  JBoss – JBoss is a pretty decent Application Server that I would have no problem using for real production applications. Consider the advantage of not having to pay license fees for every instance of a large cluster (even single machine cluster). The quickstart guide is a good place to get started. If good price/performance is needed and you're willing to fork out some reasonable $$, ,Orion is one of the leanest/cleanest app servers I've used.
• Web/Servlet Container
I've seperated this from the Application server although many app-servers come with built in web servers. In high scalability deployment configurations, the EJB's, JSP's and static files will be handled by independantly scalable farms/clusters.

Web Server performance report - an interesting resource that compares servlet container performance.

• Security - LDAP Authentication/Certificate Server
  Any large enterprise would already have an industrial strength LDAP server. For an enterprise with well-established service oriented architecture, it will also be necessary to configure an enterprise certificate authority. My default for this is Microsoft Active Directory. For the more adventurous, OpenLDAP would be the default option here.

• Integration Toolkit
  OpenAdaptor is my choice for this layer. Allows complex integration workflows to be built by configuring a set of source, sink and pipe components through a configuration file. Mercator Integration Broker and Websphere MQ Integrator are more expensive commercial alternatives.
  
SOA Foundation
JBOSS.NET supported by the XDoclet extension is a a good choice to provide a web services foundation. Understand the SOA big picture.