Brevity: The Ultrafast Text Editor
Brevity helps you save time by letting you write text several times faster. Really.
It works on the principle that most words can by uniquely identified using less letters than they actually contain and it let's you write them that way. Just start typing a word using the first letter or first consonant and continue writing any letter in the word in the proper order. Brevity will automatically list all the possible matches, which you can then select (e.g. you can write "misunderstanding" by just typing the following sequences: "msndr", "miudr", etc).
If a word is not already present in a dictionary, simply write it out and Brevity will remember it the next time you type it. It also remembers the frequency of words you use and sorts all matches accordingly, letting you write most words using 1, 2 or 3 letters.
Not only is Brevity great for native speakers of the supported languages (see the list below), but also for non-native speakers as you don't need to know the exact spelling in order to write the word. It also saves time with languages that use diacritics as you will be able to skip them in most cases.