Commentaries on Haggai

(Feel free to add your own in the list if you think I have missed something significant, or to add your comments to mine, particularly if you think I have misjudged these.)

Highly Recommended (roughly in order of preference)

1. Pieter A. Vierhof, The Books of Haggai and Malachi (NICOT) — If you only have one commentary on Haggai, make it this one. An excellent commentary that interacts with a number of different views with a conservative approach with good documentation.
2. John Kessler, The Book of Haggai — If you can afford it, get this one. It is a lengthy tome that is much more comprehensive than most commentaries (279 pp. on Haggai alone). It is among the most well-documented. I have not checked the price, but it is a Brill publication, so you might have to mortgage your house to buy it. If might be cheaper to move near a library that has it. Whichever, it will be worth it if you want to study Haggai.
3. Richard A. Taylor and E. Ray Clendenen, Haggai, Malachi — This volume is in the New American Commentary, and like most of the volumes is fairly technical, but still useful for academic study as well as pastoral study. It also is well-documented.
4. David L. Petersen, Haggai and Zechariah 1-8 — This is an Old Testament Library volume and comes from a less conservative approach. However, it interacts well on the difficult issues and will be a worthy addition for the serious study of Haggai. It is not as well-documented as it could be.
5. Alec J. Motyer, "Haggai," in The Minor Prophets (ed. McComiskey) — Motyer does a good job interacting with the Hebrew text, as well as making expositional comments on the bottom half of the page. Overall, the expositional section is better than the exegetical section since the latter is too brief. This volume is also fairly light on documentation.
6. Robert Alden, "Haggai," in The Expositor's Bible Commentary — Long enough to be helpful as an introductory commentary to make one aware of many of the difficult issues, but not long enough to do much more than assert a position on these issues.

Worth Having as Long as You Have Others (in no particular order)

1. Herbert M. Wolf Haggai and Malachi: Rededication and Renewal — A short volume in the Everyman's Bible Commentary Series. It is too short to serve as a primary or major contributor, but helpful in many ways.
2. Joyce G. Baldwin Haggai, Zechariah, Malachi — From the Tyndale Old Testament Commentary Series, it fits right in the mold of the other TOTC and TNTC as a short, basic volume that is helpful, but not comprehensive enough to interact on the more disputed issues.
3. Eugene Merrill, Haggai, Zechariah, Malachi — Merrill is a superb OT scholar with a very conservative view of the Scriptures. This is another good volume, lengthier than the previous two, with decent documentation.
4. Robert Chisholm, The Minor Prophets — This book covers all the minor prophets with a brief commentary, paragraph by paragraph through each book. It is brief, mostly undocumented, but very helpful in gaining an quick, overall perspective of the minor prophets. Overall, this is probably the best single book currently available on the minor prophets. It would make an excellent textbook for a Bible college or entry-level seminary class.

If They Are Cheap

1. Ralph L. Smith, Micah-Malachi — Part of the Word Biblical Commentary Series, it is not earthshaking, but will be somewhat helpful. It's greatest contribution is the translation and text critical notes. Overall, nothing that cannot be found in other commentaries.
2. Carroll Stuhlmueller, Haggai and Zechariah: Rebuilding with Hope — Takes a critical view and has few helpful arguments, but does have some helpful comments. Lengthwise, it fits in the category with Baldwin and Wolf.
3. Charles Lee Feinberg, Habakkuk, Zephaniah, Haggai, and Malachi — Another short work that has the added demerit of being selective. If he is addressing a passage you are studying, this may offer something. But it may skip that passage.
4. Carol L. and Eric M. Meyers, Haggai, Zechariah 1-8 — Part of the Anchor Bible, a volume of decent length, but does not seem to correlate the passage together or give a good theological synthesis. 

If You Need Kindling to Start a Fire

1. Hans Walter Wolff, Haggai — A liberal commentary that is too short to be of any value, and does not contain enough substance of liberal argument to interact with. It might not even be long enough to start a good fire.