Taskforce is a work and collaboration tool that integrates directly into your email inbox.
Taskforce allows you to create lists of tasks, share lists, integrate with GTasks and GCal; access via your smart phone, allowing: you to add and share tasks on the go.
You can add comments, dates, recurring tasks, follow ups; share multiple lists; work with people inside and outside of your company; keep constantly up-to-date with the status of your tasks and projects.
Taskforce will always have a free version.