LetMeType is a small Windows tool that extends the concept of autocomplete at the operating system's level. The application runs in the background, monitoring everything you type in any input box and creating a database of frequent words and 2-grams. When you start typing a word, LetMeType displays a list of the most probable words that start with the letters you've just typed and you can select one of the options.
The program lets you select different values for the length of the words, and for the minimum probability of a suggestion. This works well especially for long or complicated words.
The interface is not very user-friendly, but program has been open sourced and you can improve it. It would be nice to combine this with Google Suggest and to make it less annoying by only making suggestions when the user requests it or when the next letters/words are obvious.
Staying with the US Digital Service
50 minutes ago