Do It is quite possibly the most feature-packed mini-todo
widget app ever written. For a birds-eye view, it features integration with Quicksilver, Address Book and iCal, as well as .Mac syncing and AppleScript-ability. As Tim Gaden puts it: the only thing missing is a kitchen sink.
Fundamentally, Do It is (surprise) a todo manager. But thanks to shaking hands with all these other apps, you can import/export todos from iCal, install an Address Book contextual menu item for easy contact reminder creation, add items from the most excellent Quicksilver or simply make it do nearly anything you want with the power of AppleScript.
Despite all this impressive functionality, Do It remains freeware and available from Jim McGowan's site.
[Update: my bad - Do It is an app, not a widget. I tried this app out a little while ago, and in finally getting around to writing it up, I mixed this up with another todo widget I just found. Sorry guys.]