Our next intern recently asked me about commentaries for preaching, and I thought posting the reflections here might be helpful:
1) Maybe the best full commentary to purchase is the New Interpreter's Bible commentary from Abingdon Press. I own four volumes from it, but it is a nice collection geared towards preaching.
2) Each year, I buy a new commentary on the gospel for the upcoming year. For example, this year I'm reading Fred Craddock's commentary on Luke, last year I read Binding the Strong Man,by Ched Myers, a commentary on Mark, and the year before that I read Frederick Dale Bruner's two volume commentary on Matthew. Of course, John is also mixed in there, and there are lots of great options, including Luther's commentary, Origen, or Culpepper's Anatomy of the Fourth Gospel.
3) Before I left seminary, I asked my favorite bible professors to recommend commentaries. That was fun, although I learned they often weren't up on recent commentaries outside of their area of specialty, what they were currently researching.
4) At an Accordance seminar I attended recently, everyone was raving about the electronic version of the Anchor Bible Dictionary. It's now on my birthday wish list.
I do own many commentaries, although not on every single book of the bible. I don't tend to own a whole series, but instead purchase individual ones based on the author and recommendations from folks. I own a lot of the Hermeneia series, Brazos Theological Commentary, and the New International Commentary on the New Testament.
I also use a lot of on-line resources, like working preacher and textweek, and use original language source materials, on my Mac the software I use is Accordance.