DropSum is one of the better ones to arrive in the App Store and is based on a Flash game of the same name. The game also bears a resemblance to Drop7, but the goal here is to simply create adjacent numbers that add up to 9. Each numbered disc changes color as it's involved in more matches. After three times, the disc explodes and the remaining numbers fall into place potentially causing a chain reaction.
The game was well designed and includes a local high score and achievement system. The developers have promised a global high score in a future update. Check out the developer video below or check out the Flash version. Note the iPhone version only appears to have the "Basic" playmode.
App Store Link: DropSum, $2.99