Many law firms in Canada (and the United States) use Interwoven for their document management system (DMS). Interwoven is a very robust DMS that can manage large volumes of documents while keeping good version control and honoring security settings.
Increasingly, the trend is towards smarter enterprise search (Recommind, Autonomy, and Interwoven Universal Search, and SharePoint 2007 search, to name a few of the smarter search engines some law firms are using).