Codex Sinaiticus.

א or 01.

Date and provenance.
Century IV, Monastery of Saint Catherine, Sinai, Egypt.

Available scan(s).
Center for the Study of New Testament Manuscripts (index, Lake facsimile edition):

Scans of codex א.
Zhubert (select codex Sinaiticus from the dropdown menu, then select a passage).

Useful links.
Codex Sinaiticus in the Catholic Encyclopedia.
Manuscript א in the Encyclopedia of NT Textual Criticism.
Codex Sinaiticus at Earlham.
British Library catalogue entry.

Codex Sinaiticus is so named because Constantin von Tischendorf discovered it in a monastery on Sinai. Its text type is basically Alexandrian with western influences.