As you hopefully know, many jobs and even internships are looking for people with some experience, a real world portfolio and professional references. The question also comes up as to how to get those things without already having some connections. Well, volunteering is a good answer. (Another might be placement agencies and your college's alumni relations office.)
In the past, I've found great opportunities through Volunteer Match. They have tons of big and small opportunities. A search just now for my area found individuals, the Kennedy Center in DC and local government volunteering and charitable organizations. They also have virtual opportunities where you can help from your home computer. Being a print designer, i found volunteering to be a great way to solidify my web design skills. Many charitable organizations would love for you to give them an awesome new website.
You might also think about looking through listings on Craigslist. You could also freelance for work, and sure do that too. But as a volunteer you can help out some pretty worthy causes and often get a nice letter of recommendation or thank you letter. These are great for references without the hassle of dealing with the business side of freelance.