A financial services company, whose lawyers are essentially allocated by client business group, has three coordinators who orchestrate practice groups in securities, real estate and broker/deal law. A multinational bank I have worked with also has communities of interest for such topics as anti-money laundering.
If many of your lawyers are dedicated to supporting business groups, some of them share substantive law interests. A practice group that spreads useful documents, holds periodic conference calls, sponsors intranet content, assembles material from law firm and conferences, and provides expert backup for each other brings much value to the law department. (See my post of July 31, 2005 about specialist lawyers.)