A Catena is a verse-by-verse Biblical commentary of compiled comments from previous authors. The Catena Bible online automatically pulls from a large database of commentary from the church fathers and various medieval theologians, giving wise commentary on any passage. There is also an iOS and Android app for this.
The Aquinas Study Bible hosts commentary from many church fathers. Unlike Catena, this website hosts many commentaries in full rather than pulling them verse-by-verse from a database. This gives a fuller view of a commentary and the general flow of the comments of particular church fathers.
Kata Biblon is a Lexicon of the Greek Bible (Septuagint and New Testament). The format is very useful and convenient for the reader and is an excellent site for word studies.
Studylight has multiple lexicons for Hebrew, Aramaic, and Greek. While not as user-friendly and helpful as Kata Biblon, the site proves useful as a lexicon of other languages.
Blue Letter Bible is an interlinear Bible, allowing the reader to look at the original languages of a passage from multiple textual bases (Textus Receptus, Byzantine Majority Text, Nestle Aland, Masoretic Text, LXX, etc). It is an invaluable resource when looking at the original languages of a passage compared to translation or for a quick check on manuscript variants in a verse.
The NETS is unrelated to the similarly named New English Translation. The NETS is a formal equivalence translation of the entire Septuagint and is especially useful for comparing a text in the Septuagint (Greek) to a text in the Masoretic Text (Hebrew/Aramaic). The NETS is additionally an excellent translation of all of the apocryphal literature of the Old Testament and the additions to the Hebrew Old Testament books.
The Hymnary Archives are a great source of all hymnody with brilliant search functions. Tunes and texts alike are archived alongside great detail on the inclusion of each hymn in various hymnals and organizing hymns by author and composer.