carm.org
Comprehensive apologetics site covering theology, cults, world religions, and evidence for Christianity.
www.ligonier.org
Free church history teaching series, articles, and video courses from Ligonier Ministries scholars.
www.crossway.org
Free theological articles, book excerpts, and Bible study resources from Crossway publishing.
www.openbible.info
OpenBible's visual cross-reference tool showing connections between Bible verses as a graphic arc diagram.