Indent your manifests using two spaces (not tabs), as follows:

node 'monitoring' inherits 'server' {
  include icinga::server
  include repo::apt
}

Always quote your resource names, for example:

package { 'exim4':

Not

package { exim4:

Use single quotes for all strings, except when:

In these cases you should use double quotes. Puppet doesn't process variable references or escape sequences unless they're inside double quotes.

Always quote parameter values that are not reserved words in Puppet. For example, the following values are not reserved words:

name => 'Nucky Thompson',
mode => '0700',
owner => 'deploy',

But these values are reserved words and therefore not quoted:

ensure => installed,
enable => true,
ensure => running,

Always include curly braces ({}) around variable names when referring to them in strings, for example:

source => "puppet:///modules/webserver/${brand}.conf",

Otherwise Puppet's parser has to guess which characters should be a part of the variable name and which belong to the surrounding string. Curly braces make it explicit.

Always end lines that declare parameters with a comma, even if it is the last parameter:

service { 'memcached':
  ensure => running,
  enable => true,
}

This is allowed by Puppet, and makes it easier if you want to add parameters later, or re-order the existing parameters.

When declaring a resource with a single parameter, make the declaration all on one line and with no trailing comma:

package { 'puppet': ensure => installed }

Where there is more than one parameter, give each parameter its own line:

package { 'rake':
  ensure   => installed,
  provider => gem,
  require  => Package['rubygems'],
}

To make the code easier to read, line up the parameter arrows in line with the longest parameter, as follows:

file { "/var/www/${app}/shared/config/rvmrc":
  owner   => 'deploy',
  group   => 'deploy',
  content => template('rails/rvmrc.erb'),
  require => File["/var/www/${app}/shared/config"],
}

The arrows should be aligned per resource, but not across the whole file, otherwise it can make it difficult for you to cut and paste code from one file to another.

When declaring file resources which are symlinks, use ensure => link and set the target attribute, as follows:

file { '/etc/php5/cli/php.ini':
  ensure => link,
  target => '/etc/php.ini',
}