Although there are many ways for lawyers to collaborate in a secure environment using the Internet, many of the "solutions" require the use of an outside vendor or other IT professional. At the BlawgThink 2005 conference in Chicago, I learned that some lawyers have had success collaborating between firms using wikis, a solution that's both inexpensive and within the technical reach of many lawyers.
A wiki is a type of website that "allows users to easily add and edit content" and is "especially suited for collaborative authoring." A wiki can be password-protected. Although I'm using a password-protected weblog to collaborate with another firm, a wiki would give me more options for storing and sharing information. The lawyers I talked to at BlawgThink suggested that I establish an account at Hosting Matters, which enables users to set up a basic wiki with the push of a button.
I haven't done it yet, but I might. I'm also exploring other options by reading the Legal Extranet Blog, which contains information about a number of more traditional solutions to multi-firm-extranets.