I recently finished this illustration and design for the Cabot Community Theaters production of Chitty Chitty Bang Bang. All my time and work was donated to the theater to help them. This included the above design and a black and white version for t-shirts.
I believe that as an artist, I should get paid for the work I do. Generally, when someone asks if I can do free work, my response is no. I also believe in giving to others. There are many organizations out there that just don’t have the finances to hire a good designer or illustrator, but they can benefit from the work we do. 
The pro bono work that I do is mostly for not-for-profit organizations that are doing something that I can support. To date this has been largely for children’s theaters. They are trying to do so much good for so many with so little. These organizations need all the help they can get. 
One friend of mine said that doing pro bono work for not for profits was what lead him to start his own design business. The not-for-profit organizations all had board members that worked for other for profit companies. When they needed work, they were already familiar with what he did. He was then hired and paid and continued to get repeat customers.
Although this would be a nice benefit to my donating my work, it is not the reason why I do pro bono work.
There are a few times I have needed to make sure I am not taken advantage. In these situations, I have found that reminding people how much the work would be if I was not donating my time and skills to them would be has been effective. 
I value the opportunity to do pro bono work for the following reasons. 
• I know they are in need.
• I get to work on really fun projects with little art direction.
• When I am not busy with other paid work, it gives me a deadline to do something more for my portfolio. 
• I have a higher level of creative freedom.
• It gives me an opportunity to explore process, medium and style.
• I like to help others out.
• I love the organizations I help (I really love theater).
• Did I mention it is fun.