Codex Sinaiticus contains the New Testament and parts of the Old Testament. It is one of the oldest surviving Bibles. It was written on vellum (animal skin) during the time of Constantine. According to Metzger:

It once contained the entire Bible written in a carefully executed majuscule hand and arranged with four columns per page, measuring about 15 by 13 1/2 inches. Today, parts of the Old testament have perished, but fortunately the entire New Testament has survived. In fact, Codex Sinaiticus is the only known complete copy of the New Testament in majuscule script.

It was discovered in the mid 19th century at the monastery of St. Catherine on Mount Sinai which is where it got its name (Metzger writes of the history and drama of the find and what lengths Constantin von Tischendorf went to transcribe it and how it went from St. Catherine to Russia and was purchased by the British Museum).
It has been digitized and assembled and made available for the first time on the Internet. Parts of it are owned by four institutions:

The ancient parchments, which appear almost translucent, are a collection of sections held by the British Library in London, the Monastery of St. Catherine in Sinai, Egypt, the National Library of Russia and Leipzig University Library in Germany.

The codex can be viewed here.