There’s a ton out there. Here are my favorites so far:
biblegateway.com – this site I use quite a lot because you can search a passage across many translations at ONCE, and view the differences (try Passage Lookup in the left nav) – includes translations into other languages
blueletterbible.org – all in one study tool to find commentaries and word definitions, etc. Totally cross-linked and cross-referenced.
netbible.org – this one is interesting bc it is the first ‘open source’ bible, meaning that there are no copyright restrictions like with ALL other English bible translations. Also, they offer copious interpreters notes to go along w/ each verse, including alternate readings and why they made certain interpretive decisions. You can mouse over footnotes to see them below. You can also buy this translation, with all the notes, as a regular Bible. However, I did not do that because it is not my favorite English translation – there are others (copyrighted ;) that I like more, as far as readability are concerned (probably roughly equal in translation quality).
History and Background
ccel.org – Christian Classics Etherial Library has lots of good secondary docs, including stuff by the Church Fathers