IMPORTANT NOTE:
Due to the limited availability of e-commentaries from publishers, most series listed below are incomplete. Click a series title to see the biblical books available in that series.
A site that can be very helpful for looking up passages, finding Strong's numbers, and seeing a passage alongside the original Greek or Hebrew (look for the "Tools" button after a search). Be wary of the commentaries and reference resources on this site, as they are generally older, public domain sources.
A useful site for finding verses, viewing a passage in English alongside Hebrew or Greek in an interlinear Bible, and looking up Strong's numbers or TDNT/TWOT references.