A number of publications rate law firms according to various systems, and I have made observations about them generally (See my post of July 4, 2006: ranking sources of guidance on law firms; July 21, 2005: Zagat-style ratings for law firms; April 6, 2007: risks of crowd-hacking on evaluation systems; and Nov. 11, 2005: referrals and networks of firms.).
When managers in law departments want to find firms who might represent them, they can peruse these listings (See my post of June 9, 2008: various sources, including Best Lawyers, Chambers, SuperLawyers, Law Dragon and Avvo; Oct. 12, 2008: Executive Counsel and its methodology; Aug. 14, 2006: ratings of law firm websites; Jan. 25, 2008: Martindale Hubble ratings; Feb. 7, 2007 on several rating systems; Nov. 19, 2007: InsideCounsel offering; and Sept. 10, 2005: law firm size and ratings on league tables.).