Mail, like almost all Cocoa applications on Mac OS X, offers text completion, which can be invoked by hitting either F5 or Esc. When it was introduced, it offered completions based on the list of words from the mail you were typing. Unfortunately, somewhen Apple decided that it would be smart to use the system dictionary for completion. From that day on, using the completion in Mail.app meant to have to scroll through an endless list of words to finally get to the one you wanted. In other words: it slowed typing down instead of making it faster. And it was (and is) absolutely useless when writing in a language that's not your system language.
Enter Ofaco. The name "Ofaco" stands for "Old-fashioned completion", which says it all: it's a plugin for Mail.app that replaces Mail's dictionary-based completion mechanism with one that uses the words from the message window from which it was invoked. Additionally, I added a way to define default completions that will always be added to the completion list (and which, by the way, can not only contain words, but also phrases).