Who doesn't like free? The Hopiko game is a fun little speedrun platformer with hand-crafted levels.
There's (of course) a tiny plot where you, the hero, don power gloves to save gaming from the evil Nanobyte virus. It also boasts unique touch screen controls and a cute chiptune soundtrack.
Speedruns means that you'll have to race not to contend with the various obstacles but against the clock. Each level has a set goal time that you can try to beat for that extra challenge, which gives it plenty of replay value for the more competitive of you. It always gives you something new to beat if you're also the type that likes to wrack up high scores.
What makes the game a challenge is that the levels are arranged in five level runs, where you must complete those five levels to advance. There are even other challenges such as stages with not five, but 10 runs.
Hopiko is a universal app compatible with most iPhones and iPads.