I know this is old news, but I thought it should still be captured here.
The old International Critical Commentaries are available online now that the copyright has expired.
Why you would want them: If you are keen with the original languages, or you know enough to ask tough question that might not get answered in your typical commentary, then these scans will come in handy.
Commentary Leanings: Almost all of these are going to be pretty liberal. If you've ever been curious as to who had a better vocabulary, first or second Isaiah, or if you believe knowing how J and E used to pick on D and P in handwriting class will help you interpret the Pentateuch, then these commentaries will surely answer all of those questions.
Bummer Factor: These are not OCR'd, so you can't search within the PDF, but you can also download the "Full Text" through Archive.org. This is what I do: Download the scan because that is far nicer to read and cite from, but also download the full text. name it the same thing (except .pdf and .txt respectively) so that when you decide you need to use the recourse you can open the .txt file in Word or OpenOffice and do all of your searches there. That also allows you to copy and paste directly into your paper. It's an only slightly annoying way to get around the lack of OCR. *Update* almost all of the Archive.org pdfs have been OCR'ed, so download them there.
Check out the list of OT commentaries and NT commentaries.