I created Stuff To Do (stufftodo.dedasys.com) as something we needed for work, but on my own time, so I decided to experiment with it a bit. I came up with what I thought was a pretty good separation of pay/free versions: the group-enabled version costs money because if you're working in a group, you're probably in a company, and companies have money...
However, it is also a space that's got a lot of people in it, and while I think I have a few compelling features (the time tracker is the easiest out there) free is tough to beat.
Not sure what I'll do with it, because it's not really the sort of application that lends itself well to advertising (I'd find it pretty annoying to have ads on my todo list).
Multiply the increase by the amount of users that receive it and the number doesn't seem so small - and can never be considered 'no cost'. The trade off is that with the more mail they can index per user, the more relevant advertisements they can display and the more people they can lure with their gimmick. Of course, they're also taking advantage of the 10% rule when it comes to offering that large amount of storage.