In the European Union we have a "Common European Framework of Reference for Languages". In practice all language certification programmes have one of those levels associated to them (ranging from A1, the most basic, to C2, the most advanced), so that it is possible to establish equivalences between common tests. This allows me to, say, determine if my A in the (life-time valid) FCE exam is worth more or less than a 8.0 in the (short-lived) IELTS exam.
This however doesn't help the student that wants to have an exchange program outside the European Union. I heave heard rumours that, e.g., the Cambridge exams in English aren't accepted in the states, but that could very well be FUD.
What are the most universally accepted certifications in English?