Android: It's one thing to stay on top of your own finances, but when you're sharing bills with a roommate things can get extra complicated. Conmigo is an Android app that seeks to solve that problem by splitting up expenses in any way you choose and always letting each person know exactly how much they owe (and when).
You're not required to use this app with just a roommate, too. It'll work fine with a significant other or anyone with whom you're sharing expenses.
Like any personal finance tracker, you add your expenses and sign them to a category. In the case of Conmigo, you can also assign them to a person. When you're ready to mark things as paid, you can use the Settle Up screen to do so. It'll let you mark individual items or clear them all out at once. You can also see your history if you're not sure who paid what.
Conmigo is currently free on the Android Market, so if you're in need of a multi-person expense-tracking app you should go check it out.