I've got an N900 on T-Mobile in the US, and while most things are unlimited, but not everything, especially roaming.

I'd love something that would alert me any time I spend significant money on this account; T-Mobile has alerts only for voice stuff, which is pointless really.

A widget that showed my actual balance would be cool.

A service that sent me an SMS or email would be fine too.

No idea if such a thing exists, but I'd pay for either.

-Robin

a service sending you a SMS may be offered by t-mobile. this has nothing to do with the n900.
check the online-portal of t-mobile and see if you can activate a sms-service that warns you when you go over a specific value.

my provider offers sms when the bill goes over 5, 10 or 20 euro. you have to set that up by your provider.

/edit:
but yeah, some kind of customizable desktop-widget would be nice too.

I already said that T-Mobile's service for this purpose is totally insufficient fro me; it doesn't alert on data charges or SMS charges, only voice.

But it *could*. That's the nice thing about having a general computer instead of just a phone.

I'm actually a little surprised no-one's already done this as a web-based service.

-Robin