Christine has been using Remember The Milk for a while at work. It's a to-do list service that has been around since October 2005, but their continual development and integration into iPhone, Blackberry, Gmail and others has fueled its growth to over 600,000 users.
I recently caved in and downloaded its Gmail Firefox extension, and loved it. I like it that my to-do list is shown side-by-side with my email. I go through my email often, so it's good to have my daily reminders in front of me or I can easily forget when my schedule goes crazy.