Remember the Milk
On the productivity side, you cannot beat Remember the Milk for simple functionality.
I use this one to keep track of everything I need to do. Functionality is basic, but covers everything a to-do list needs to do and then has a number of useful features.
My favourite, and the one I use to sell it to my colleagues, forms part of my workflow. When you register you are assigned a custom email address. You can then mail tasks to yourself. So, as I process my inbox, I literally forward things I need to deal with to myself. Using a basic syntax, you can assign the mailed task to a specific list, give it a due date, tag it, and a number of other features which I don’t necessarily use very often.
My next step is to upgrade to pro so that I can have my tasks automatically synchronised between RTM and my new BlackBerry