I preached through the book of Acts in the early 90's while at Christ Community Church in Ardmore, Oklahoma, and hope to do so again. At the time, I struggled to find good commentaries. Since then, however, several excellent resources have become available. I'll begin with works that I used in sermon preparation and then move on to those that have been published within the last decade or so.
If you are looking for a commentary on the English text, you can do no better than John Stott, The Spirit, the Church, and the World: The Message of Acts (IVP, 1990, 405pp.). Stott is superb when it comes to tracing the argument of the book and making relevant application to church life today. I didn't always agree with Stott on passages dealing with spiritual gifts, but he is excellent and should be in every pastor's library.
Also quite helpful for those who don't read Greek is the commentary by I. Howard Marshall in The Tyndale New Testament Commentary series (IVP/Eerdmans, 1980, 427pp.). Most of the volumes in this series are painfully brief, but Marshall's is an exception. Get it.
Another extremely helpful treatment of the English text is by F. F. Bruce, The Book of Acts, revised edition, in The New International Commentary on the New Testament series (Eerdmans, 1988, 541pp.). Don't confuse this one with Bruce's commentary on the Greek text. There are better alternatives to the latter, as noted below.
One more good resource is the commentary by Richard N. Longenecker, The Acts of the Apostles, in The Expositor's Bible Commentary (Zondervan, 1981, 366pp.). Another popular treatment on the English text is the volume by Ajith Fernando in the NIV Application commentary series (Zondervan, 1998, 656 pp.). It's probably worth a look, but I haven't had the opportunity to read any of it as yet.
More recently several excellent treatments of the Greek text have become available.
Perhaps the most technical and exhaustive is the two volume work by C. K. Barrett, The Acts of the Apostles, in The International Critical Commentary series (T & T Clark, 1994/1998, 1272 pp.). In all honesty, it is almost too technical for the average pastor. I appreciated the depth of analysis he provides, but I was often frustrated by the lack of an equal commitment to addressing the theological and practical issues that arise in the book.
I would recommend that instead of Barrett (by the way, the two volumes are very expensive) one purchase the new commentary by Darrell Bock, Baker Exegetical Commentary on the New Testament (Baker, 2007, 848 pp.). Bock will provide all the exegetical information you need together with insightful treatment of the theology of Acts. If you are going to invest in his two-volume treatment of Luke (and you should), this is an excellent companion volume.
A couple of years ago I came across an interesting new series that is devoted solely to grammatical analysis of New Testament books. The first one I saw was Martin M. Culy and Mikeal C. Parsons, Acts: A Handbook on the Greek Text (Baylor University Press, 2003, 558pp.). There is virtually nothing said about structure, argument, or theology, as the authors simply provide parsing and grammatical observations on the Greek text. I found it quite helpful.
Although I haven't used it much, the contribution of Joseph A. Fitzmyer (a Jesuit priest) to the Anchor Bible series, The Acts of the Apostles (Doubleday, 1998, 830 pp.) is certainly worth a look.
There is one commentary that I highly recommend but find hard to classify: Ben Witherington, III. The Acts of the Apostles: A Socio-Rhetorical Commentary (Eerdmans, 1998, 874 pp.). This is a massive work that provides a wealth of material for the student and preacher of Acts. You don't have to know Greek to profit from it, but that doesn't mean Witherington ignores essential exegetical details.
Finally, although not a commentary, I highly recommend the collection of essays on Acts found in Witness to the Gospel: The Theology of Acts, edited by I. Howard Marshall and David Peterson (Eerdmans, 1998, 610 pp.). Some twenty-four contributors have provided a wide range of essays on virtually every important topic in Acts. They aren't all of equal value, but this is a great resource to have ready at hand.
In conclusion, if you are getting ready to preach or teach (or simply study) through Acts, I would recommend, in order of preference, Stott, Bock, Witherington, Marshall, and Bruce.
Romans is next!