As I preach through the book of Hebrews, I have been consulting most of the top commentaries. They all have their strengths and weaknesses. Some are stronger on the Greek text, but less on the explanation of the text, or the application.
John Owen's Hebrews commentary however has been a joy to consult. His commentary not only examines the Greek text, but also the logic of the author's arguments, and makes good observations and applications on these.
You can get hold of this commentary the following ways:
1) Purchase the 7 vol book set somewhere.
2) Download free scanned pdfs of it: http://www.archive.org/search.php?query=%22John%20Owen%22%20AND%20collection%3Atoronto
3) Purchase Owen's whole collection as a pdf for £8 from: http://www.ageslibrary.com/ages_owen_collection_1.html
This is much easier to search through than the free scanned pdfs - but doesn't give you orignal page numbers.
4) Purchase it through logos (I think its on pre-pub at the moment).
Of all these, I like the pdfs the best, because now with acrobat reader you can highlight and underline text. Plus I find Libronix gets busy enough with all the things I have running on it, so I like to have Owen open in a seperate application.