Tapfish is a game where you can buy, raise and sell fish - like a tamagotchi, all grown up and moved to the smartphone platform (and prettier!) You can play quite a bit of the game without paying for anything - so much in fact that I got quite into it. When you consider that I've played daily for 6 weeks, 10 quid for the add-ons that will let me play more of the game seems reasonable.
Often these types of games are quite aggressive in the way that they take your money from you - you can only play a little bit, or the game just stops completely until you pay. This one though uses a nice combination of gamification and playability to bring you to the point where you don't resent parting with the cash.
With any freemium model, you will always have users who will never spend money at all, and people who complain that it's a rip-off. Striking the balance however is really tricky, and honestly? I think this game got it right.
* when I say quite a bit, I mean I'm completely addicted to tapfish and have played it every day even including getting married and going on my honeymoon!! So yes, quite playable.