There are quite a few in the marketplace...I don't have time to try them all. You can't search on "to do list" on the forums...is there already a thread that compares some of the available widgets for this?
Astrid is quite good. As a bonus, if you use Pure Calendar Widget (which looks very nice), there's a version of Astrid that it syncs with, giving you your to do list before the upcoming appointments. Pure Calendar syncs with google cal, but I don't use Google tasks so I'm not sure about that.
I swear by Astrid. I use Remember the Milk at work and it syncs perfectly with Astrid and Google calendar.
Aside from actual work stuff, I put grocery items in and tag them groceries. when i am at the store, i can pull up all the items tagged groceries and just check them off one by one.