Project Agape who is behind the Causes Facebook app uses the following:

    *  mySQL 5.0
    * Ruby on Rails
    * 14 application servers, 2 database servers and 5 boxes in reserve
    * 3 Image Servers (2 Squid, 1 image source)
    * Apache 2 on Image Source
    * WebApp servers have 2GB of ram
    * 1 server for serving static content
    * 26 Mongrel services per box
    * 4 GB of RAM per box
    * 50 - 60 MB of RAM per Mongrel service for a total use of around 1.5 GB of memory set aside for Mongrel

Additional:

    * A pair of F5 Loadbalancers
    * All systems running FreeBSD 6.2
    * memcached
    * nginx (not pound)

Specific Rails technologies used: nginx (not pound), Evented Mongrel, squid From: http://www.nonprofittechblog.org/interview-with-joe-green-an...