The standard edition of the Hebrew Text of the Old Testament is the Biblia Hebraica Stuttgartensia (BHS), which follows the Masoretic Text (MT).
Here is a guide to using BHS, with notes on how to read the apparatus and abbreviations.
The Septuagint was a Greek translation of the Hebrew Bible made in the 2nd century BCE. It was in common use throughout the New Testament period and Early Christianity. The resource below is a standard edition of the Greek text.