Having personal projects on the go, in addition to your client work is very important for your development as an artist. If all you do is client work (work for someone other than yourself), you will probably get burnt out, and maybe even discouraged with your work. With personal projects you are your own client, and you can create work that you want to create, without having to change it for someone else.
The majority of what I have in my portfolio right now is personal projects, as I haven't gotten too many clients yet (working on it!). And that is okay. It shows self-discipline, and pursuing what you are interested in, it shows part of your personality. One ongoing project I have going on right now is my Tarot card project. This is a project that I started while in school, and have decided to keep working on it. I am interested in exploring the deeper meanings and symbolism behind the cards. They are also handy as self-promo pieces, and I have sent them out to art directors as well.
Another form of personal projects is to create work for the field you want to work in. For example, if you want to work in the book publishing industry, create book covers to put in your portfolio. If you want to work in editorial, pick out some magazine articles and illustrate them, create spot illustrations. When I graduated from school, I didn't have many spot illustrations in my portfolio, so one thing I did was create spot illustrations to flesh out my portfolio.
It's always good to have both client and personal projects on the go, so you can feed your wallet as well as your soul.