* sendmail-milter

  If you run sendmail, this milter will plug right in.
  You don't need to run spfd.

  You do, however, need to give the milter an "mx" argument
  if your MTA serves as the primary MX for domains which
  have secondary MXes that you don't know about.  When a
  secondary MX relays mail to you, you want to bypass SPF
  checks.  With an "mx" argument, the milter will look at
  each recipient's domain and detects if the connecting
  client is a secondary MX.  If you don't understand what
  this means, you probably don't need to do this.

* exim-acl

  If you use Exim 4, this ACL will plug right in.
  You will need to run spfd for this ACL to talk to.

* postfix-policyd-spf

  If you run a recent version of Postfix, this policy server
  will plug right in.  You don't need to run spfd.
