E-Sword Bible Software — this is a great tool. Download a free Bible program that lets you search the Scriptures, view commentaries and other tools, etc. Rick Meyers has hit a home run. Donate if you find it valuable, but it is free if you can’t afford a donation.
Blue Letter Bible — Very nice Bible search website. Also extensive study aids — commentaries, maps, charts, etc.
BiblicalStudies.org.uk — Great resource for theology students. Many theological works available. Read with discernment, some resources are doubtful.
BiblicalPlaces.com — Great pictures of places mentioned in the Bible. A large collection of more pictures for sale beyond what is visible on their website.
P-R-E-C-E-P-T A-U-S-T-I-N — Commentaries, commentaries, and more commentaries. Greek tools, Bible dictionaries, maps, etc. Not everything here is entirely good, but there’s a lot that is.
Treasury of David — Actually, that’s not accurate. It’s the whole Spurgeon Archive. But this particular page is the Treasury of David, because out of all the great stuff by Spurgeon, this is what I most wanted to link to.
Fire and Ice — Great resource for Puritan/Reformed writings.
Sermon Audio — Thousands of sermons.