Tricount - Manage and share expenses with friends

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

Kittysplit - The easiest way of sharing costs with a group of friends

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

ihatemoney - Manage your shared expenses, easily

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.