NA27 is no longer available. However, Nestle-Aland Greek New Testament, 28th Edition, with Critical Apparatus is available for purchase.
Novum Testamentum Graece is the Latin name of the original Greek-language version of the New Testament. The first printed edition was the Complutensian Polyglot Bible by Cardinal Francisco Jiménez de Cisneros, printed in 1514, but not published until 1520. The first published edition of the Greek New Testament was produced by Erasmus in 1516. Sometimes referred to as the "critical text," this is the Greek text most widely used today. It is the basis for nearly every modern Bible translation in the past one hundred years. Also included is the Nestle-Aland Novum Testamentum Graece Apparatus Criticus.
It really is a pleasure to look up and search in ways never thought possible before these great electronic resources. I must confess that the 'wow' factor remains high even after roughly three months of use.
—Rubén Gómez, Bible Software Review