While the idea of extensive reading as a powerful language learning tool has been around for at least a hundred years, it seems that recently, with the Internet making authentic language material only a few clicks away, more and more people have discovered this most enjoyable method of acquiring language. Language learning, we have been informed, is supposed to be hard work; so it almost seems like cheating to learn easily and effectively by doing something we'd like to be doing anyway - only in the target language. But it is after all only reasonable that if every other skill improves with practise, the more practise the better, reading ought to follow the same pattern; and if the practise doesn't feel like work, more practise will happen.
Extensive reading, to be exact, refers to reading large volumes of material at or very slightly above one's comprehension level, aiming for overall understanding of the work instead of worrying about every detailed twist of grammar or every word. This approach stresses learning from context and getting used to the language by massive exposure instead of understanding things point by point. It does not replace its opposite, intensive reading (in which a difficult point or a sentence well above the learner's level is studied in detail); but rather is a vital supplement. In fact, intensive reading and study should instead be viewed as a supplement to extensive reading, which turns the normal classroom paradigm on its head.
The four skills of reading, listening, writing, and speaking ought to be viewed in that order, because each one feeds the next. You can't say what you don't know, and reading is much easier to learn words from than listening. The large quantity of exposure helps the learner to think in the target language, which in turn improves output abilities. Many case studies have firmly established the effectiveness of this method. For more details, read Rob Waring's The Inescapable Case for Extensive Reading, or follow some of the links from www.extensivereading.net.
The ReadMOD contest was established to promote this least-method-like of methods. It is extremely simple: read every day for a month, and report your pagecount; the person with the highest pagecount at the end wins eternal* fame and glory, the adulation** of the other contestants, and notably improved language skill.