I was listening to Mark Driscoll's sermon online from a week or two ago and he was encouraging people to study and learn their Bible. As one of the resources he mentioned a glorious site that rates a ton of the commentaries out there. I don't know how it all works in terms of ratings and what not, but it has potential to be a great help, particularly in these times where there's less money to waste on a mediocre commentary. The site: http://bestcommentaries.com/.
In the past, and I'll likely still use it, I've used Tremper Longman's Old Testament Commentary Survey for finding my OT commentaries and the Top Picks put out by the Talbot School of Theology staff for my NT commentary choices. Both are, at least until I figure out how to get more reviews out of bestcommentaries.com, more helpful in giving reviews - even if just a few lines long.
Enjoy. Or roll your eyes if you can't believe what an incredible nerd I and my ilk are.