Producteev is an online organiser and to-do suite that incorporates a comprehensive alert system including email, instant messaging and mobile apps, along with the ability to sync to Google Calendar effortlessly.
With the ability to sync each task's due date to Google Calendar, Producteev integrates the two organisational tools and eliminates one task for the average planner. Each task can also be assigned to different people, and tagged with labels for easy searching.
After connecting your email accounts and instant messaging systems, Producteev makes it easy to add tasks from various amount of methods. You can add a task via AIM for instance, or by sending an email to Producteev. Conversely, Producteev alerts the user through any delivery system that is set up for the task. Instead of getting an email, why not get a notification through GTalk instead?
Producteev doesn't have the same kind of following that organisational tool Remember the Milk does, but it seems like a pretty comprehensive alternative.