I had Predictive in my list of Emacs packages to try out and review, but Mathias Dahl has saved me the trouble by writing a nice tutorial on using Predictive, complete with screenshots and all. In a nutshell,
Predictive is a package for the GNU Emacs Editor that helps you write text faster. It does this by trying to predict what you are going to write by looking at the first few characters of each word you type, looking into a dictionary for matching words, and suggesting words from this dictionary.
Mathias goes on providing all the details you need to get stated with Predictive. Although i have not yet used it, looks pretty interesting.
Oh, as an aside, i recommend taking a look at Tony ‘Qubit’ Cubitt’s site for more Emacs (and non-Emacs) goodies, or just to enjoy a beautiful website.