Services you'd keep using if they weren't free

There are a lot of services I use because, in addition to being useful and convenient, they are free.

Despite how much I may gain in productivity or enjoyment from a service, it may stop being so as convenient or approachable when I have to grab my credit card to check it out. But that said, if Google Calendar, Flickr, or Wordpress became pay only, I probably would take the time to sign up for a subscription account.

For those of you entrepreneurs with non-pay business models: Would a user want to continue using your service or product if you began a mandatory charge? How about those of you who already plan to charge: Would your user-base increase x-fold if you decided to make it free?

I think these might be some interesting questions to ponder when you're still in the early stages.