The Codex Sinaiticus is significant because 1) it was copied comparatively close to the time of the original autographs. 2) It is the earliest complete collection we have of the entire New Testament canon. 3) It reveals much about the ancient copying and editing processes which preserved the Bible text we use today.
It was discovered in the 1840′s and 50′s by German scholar Constantine Tischendorf in the Monastery of Saint Catherine, which sits at the foot of (traditional) Mount Sinai. He brought portions of it to Germany in 1844 and 1853 for publication. Another large portion went to the Russian Tsar in 1859. In 1933 the Soviet Government sold most of its collection to the British Museum. In 1975, more of the Codex was discovered at the Monastery.
Today the Codex is split between the British Library in London, the University Library in Leipzig (Germany), the National Library of Russia in St. Petersburg, and Saint Catherine’s Monastery (Egypt). Efforts to physically reunite the Codex in one place have not succeeded, but The Codex Sinaiticus Project has digitally united all of the manuscripts online. For the first time, scholars (and, well… anyone) can view every part of the Codex Sinaiticus.
The page to view the actual manuscripts was overwhelmed when it opened today (July 24) but the web traffic should be manageable soon.
[Update] The page is accessible now, but the entire Codex is not yet available. More manuscripts will be added over the course of the next year. The site is nice. You can zoom in and see corrections and erasures, and you can do book/chapter/verse searches of the text.