I think trials can be improved for services where you can discern whether the user is directly interacting with it. You give them 100 days of usage, and then decrement a counter when they interact with the site the first time each day.
(edit, removed indefinite pronoun)
Also, please don't send multiple "You've only got X-N days left in your plan!" pressure-to-convert emails.
Daily quota is the way to go. Amazon free micro instances are a perfect example. Enough to build. Not enough to go to production.