This is one of those games that's more difficult to explain than it is to play. The basic premise of Drop 7 is that you drop blocks into a 7x7 grid. Every block has a number on it. When the number on the block matches the number of blocks in a row or a column, it disappears. When people try it, they tend to want to liken it to a blend of Tetris and Sudoku, but it's really not that complicated. Aside from being hellishly addictive, it's also aesthetically exquisite. The elegant, minimalist graphic design is perfectly matched with a spacious soundscape and beautifully appropriate sound effects. It really is a complete aesthetic experience, as well as a real brain workout.
UPDATE: Drop 7 currently experiences some issues with iOS 4, and the development team at Area/Code have submitted a fix to Apple. Pending approval, the update will be live any day now and the game will work again.
The Best iPhone Games
Top 9 iPhone 'Twitch-Action' Games
What iPhone 4, IOS 4 Mean for Games