(While this has been covered elsewhere fairly recently, I'm posting this here so I don't forget it. The principle is basically the same as JDD's, except neatened up slightly)

With all the different systems that I potter around with Rails on, it can be a pain having to tweak database.yml each time you check your app out onto a different development system. It's time to be a proper programmer, end the drudgery¹ and solve this issue:

common: &common
  adapter: mysql
  username: root
  password:
<% socket =  ['/opt/local/var/run/mysql5/mysqld.sock', # darwinports
              '/opt/local/var/run/mysqld/mysqld.sock', # darwinports, again
              '/var/run/mysqld/mysqld.sock',           # ubuntu/debian
              '/tmp/mysql.sock'
             ].select { |socket_location|
                File.exist? socket_location
             }[0] 
   if !socket.nil? %>
  socket: <%= socket %>
<% end %>

development:
  <<: *common
  database: engines_development

test:
  <<: *common
  database: engines_test

Some notes:

• On your production database, your login details better not be root/empty_password. Common sense, really.
• I've tried to cover the socket locations that I bump into normally, but you might have a different one. Obviously, just add that to the list.
• I wonder if there's some way to transparently hack the app generator to use this...

1: Yes, I'll admit it. Most of my programming stems from the fact that I'm lazy and don't want to repeat things. Computers are our slaves! Huzzah!