When targeting any corporation for freelance work, most writers contact communications department. It makes sense, after all, that department responsible for majority of a company's external and internal communications would have greatest potential need for freelancers.
But having worked in a large insurance company for over 15 years, I can vouch for fact that need for strong writers exists throughout organization, not just in communications. And management in these other departments often realize that need. But they probably won't even think to seek out a freelancer--unless you make them aware of your availability, and show them how you might help them solve one or more problems within their areas.
Moreover, this fact doesn't apply only to insurance companies. Utilities, banks, hospitals, brokerage firms, and a wide variety of other organizations frequently have similar departments, with similar needs. You just have to know where to look, and whom to contact.
The following list, although by no means exhaustive, contains corporate "roads less traveled" where enterprising writers might be able to pick up freelance work:
Customer Service Departments
I list this one first, only because I've spent a good deal of my own work history in customer service sector, so I know firsthand what writing needs exist there.
A certain amount of service for many companies is done via correspondence--either through form letters or more personalized responses from a representative to a customer. Often these form letters could use a professional writer's touch, or these representatives could use business writing training. Some organizations might even hire a writer to assist them with their correspondence backlogs, or with creation of new form letters.
Another idea would be to offer employee newsletters targeted to customer service departments. The newsletter might include Service Employee of Month, customer service tips and articles, motivational quotes and stress-reducing techniques and exercises. If you've ever worked in a customer service department, you'd understand how valuable this type of newsletter could be in terms of employee morale.
Human Resources Departments
Next to communications, HR departments probably produce most documentation within an organization, and most likely would be open to assistance in doing so. From developing company policies to job descriptions, from posting fliers announcing upcoming company events to handling employee benefits paperwork, human resource professionals for whom writing may not come easily need to constantly communicate to rest of company in writing. Can you help them get their messages out clearly and professionally? If so, freelance opportunities await.