iOS (jailbroken): iOS' autocorrect feature can be confusing: you have to tap the autocorrect window to ignore the correction, which just doesn't make sense. ManualCorrect flips this behaviour to something more intuitive.
I've been using the iPhone for a while and my instinct is still to tap on the autocorrect box in order to make that correction. Instead of frustratingly deal with iOS' autocorrect shenanigans, I've installed ManualCorrect, a jailbreak tweak that reverses this behaviour. Now, when I mistype a word, a box pops up with the correct word and only types the correction when I tap on that box. It isn't for everyone, but if you still find iOS' autocorrect a bit hard to get used to, ManualCorrect is great.
ManualCorrect is a free download for jailbroken iOS devices. You can also grab the 99 cent Pro version, which adds a settings pane and an SBSettings toggle for the tweak.