A collaborative program with math was not the first thing I thought of, but I came across a program on iEARN that does just that. There are many different things you could ask the students to do, but the idea is that they document how they use math in their family or community and report on it. This is great because it makes math real to them and how they use it everyday. The collaborative part is even better because they will be able to see how students on the other side of the world may use math in the same way or how differences in their daily lives may greatly change how they use it.
Objective: TSW understand how they use math on a daily basis by developing word problems that they will then trade with a student from the collaborative classroom and solve.
The students could use Google Docs so both classes have access to the problems and once they are finished they could have a skype conference to discuss their math problems and how they use math in their lives. The program is working with two teachers from Puerto Rico and one from Argentina, if they are still interested in the program it would be great to work with them.