How 2 people started Remember The Milk (simple todo list with 300K users)
blog.rememberthemilk.com
blog.rememberthemilk.com
Oh, wait -- you were serious? Because I wasn't... I will feel a teeny tiny bit bad for these guys if Google doesn't buy them, because it's so obvious that acquisition is their desired endpoint. Not that they haven't created a desirable product for people to use and Google to acquire, mind you -- it just doesn't seem like any sort of an afterthought.
Tudulist stresses the "tool to work together" point btw.