I have an Android (Samsung Vibrant, woop woop), and I love browsing the market for new apps (that are FREE. If it's not free, I'm probably not going to get it). Some of my favorites:
I use this app to remind myself of upcoming assignments, tests, projects, appointments, etc. Great user interface, and it can be synchronized with Google Calendar (which makes adding a bunch of tasks via a computer then adding them to your phone a lot easier).