Rope 'n' Fly 2 [iTunes link] is an example of a casual iPhone game done right. The gameplay is incredibly simple: you tap on a building to shoot out a rope and attach it, then tap again when you want to release it. The goal is to use your swinging momentum to move forward in the game, but also to increase your altitude since you are constantly falling.

For such a simple game, there are a surprising number of game modes to choose from. Timed modes, distance modes, even silly ones like "how far can you get with only one rope?"

But what really enhances the game is the high score and achievement functionality that is powered by OpenFeint, a social platform for iPhone game makers and players. Rope 'n' Fly 2's gameplay is solid, but very simple and repetitive. OpenFeint brings achievements, high score tracking, leader-boards, and the ability to compare your scores with friends playing the same game. In Rope 'n' Fly 2, this means it will tell you when you've successfully completed an achievement like flying more than 200 meters.

I love that you can pull up Rope 'n' Fly 2 quickly, and have a fun gaming session that lasts only a minute or two. For a price of $1, this makes it the epitome of a casual game. If you're not interested in tracking your score, or any of the slight variations on gameplay, you might be satisfied with Rope 'n' Fly 2 lite [iTunes link], a limited version of the game.

Gravitex 2 is a classic gravity-based time waster. You pick a direction and amount of power, and shoot a ball off into space where various orbs' gravitational pull affect its trajectory. The goal is to collect all the green coins and get the yellow ball safely to its goal.

Interestingly, the green coins are not persistent; if you collect one and get the ball to the goal, that coin does not return again if you replay the level. That means that you don't have to come up with a single magic trajectory that will collect every coin in one go, but instead you can tackle a level multiple times in multiple ways to try to conquer it.

The graphics in Gravitex 2 are simple but do the trick, which can also be said for the soundtrack. The game offers both mouse-based control, and keyboard control. Given that this is very much a trial-and-error based game, you'll want to get comfortable with the keyboard controls, though for very tough levels the only way to finely control the settings is to use the mouse.

Gravitex 2 is pretty much a perfect casual game for people who enjoy physics games. You can play it for five minutes on your coffee break, and come back to it for a few minutes on your lunch hour.