STA Travel is a major student organization and while they do not always have the best prices on flights, tickets purchased through them are often much more flexible and have far fewer restrictions; sometimes even refundable for a small fee. This can be great for trips without specific return dates or when travelling until your money runs out and its time to head home to make some more travel cash. STA travel has offices around the world. I've also used StudentUniverse.com to book some flights at a better price than I could find elsewhere.
Some of you might even remember Council Travel, which was one of the largest student travel agencies before they shut down. I could get one-way tickets to or from Germany from them for $150 inclusive even if the departure date was only a few days away! I did get a one-way ticket from Detroit to London to start a RTW trip for $199 inclusive from StudentUniverse.com so good deals are still out there.
STA travel runs some good deals on Round-the-World flights also!