The concept of freemium is well established on the net. Most tools and services have a free version with limitations (limited functionality, advertising etc) and then a premium version with all the bells and whistles and minus the ads at a price. The free version is the bait to encourage you to go premium once you get hooked on the service. I use lots of these services but only in a few cases have I taken the premium option; you can't afford them all.
Via Alastair Creelman