Ofaco stands for "Old-fashioned completion", which is 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, it offers a way to define default completions that will always be added to the completion list and which can not only contain words, but also phrases.
Mail.app,
Mac OS X 10.5 or later
To use Ofaco on Snow Leopard, you have to run Mail.app in 32bit mode (use checkbox in Mail's Finder information window). I hope to find the time to eventually solve the 64bit issue soon.
GCOGHILL I'm having a hard time envisioning what this app does differently than the default Apple Mail behavior (I haven't installed it). (Version 1.2)