If you're a student, or just interested in volunteering abroad, here is a link to a website where many opportunities are posted.
http://www.goabroad.com/volunteer-abroad/search/nursing/volunteer-abroad-1.
I'm not vouching for any of the organizations listed, I don't know them, but when I tell my stories many people want to know how they can become involved. So here's a stab at helping out. If you already know an organization that does work abroad, such as your church, or a local charity check with the.
Otherwise, carefully check out the organizations you're find interesting. Check their reputations, how many people have volunteered with them in the past, how long they've been in existence, the amount of money you will pay, where you will stay, and especially safety and security. Just because you travel with a kind, generous and honest heart, doesn't mean you'll be loved in return. American's are not always welcome abroad. So, do your best to make sure your skills fit the opportunity, and your vision matches the organization you go with, and that your basic needs, and security is considered.
That, is my best advice.
That said, If you do sign up, and find a place that matches your skills and abilities, values and beliefs. I'll take a line from the peace corps... It's the hardest job you'll ever love. Pay it forward big-time. Pay it forward.