I'll shamefully admit that I never jumped on the Where's My Water
bandwagon when I should have. Deep in my heart I knew it was an enjoyable puzzle experience, but I couldn't get myself to agree to the 99 cents. I don't know why. I've spent more on worse.
Anyway, that game and others are free today to celebrate the App Store's 5th birthday. There's an article about it outlining a few more games and apps.
Article: The Verge