Dr. Thomas Constable's Notes on the Bible
(Scroll down to Hebrews -- free pdf download)
Dr. Jack Arnold's teaching notes on Hebrews. (Free online, downloadable.)
The New International Greek Testament Commentary (NIGTC) The Epistle to the Hebrews, by Paul Ellingworth. (Book, technical, excellent.)
F F Bruce is always good on the text and background. (Book.)
Various commentaries on Hebrews, online, free, verse by verse.
Commentary on Hebrews, by John Calvin. (Online free, or download as pdf for small charge.)
Ligonier's top 5 commentaries on Hebrews (blog)
Dr. S. Lewis Johnson's teaching series in Hebrews (at Believers Chapel in Dallas, TX)
John Piper's messages on Hebrews.