Google has a Google Labs project available called Google Scribe. Google Scribe provides text autocompletion as you type. It provides related word or phrase suggestions, using information you've already typed into a document. 

"In addition to saving keystrokes, Google Scribe's suggestions indicate correct or popular phrases to use," says Google.

Google Scribe will show suggestions as you type by default, but that can be changed to "on demand" or my selecting an "on tab" option in the toolbar. Functionality can be toggled on and off with a keyboard shortcut. 

Users can choose if they want suggestions to be sorted by alphabetical order, "Google Scribe Score", which is based on popularity, relevance, etc., relevance, or expected typing savings. 

There is a Google Scribe bookmarklet that lets you use the tool on any web page. You can give Google Scribe, enable/disable it, and add the bookmarklet here.

The Google Labs listing indicates that Scribe was last updated last month, and I have seen a couple mentions of it from the past, so it appears that it's not a completely new product, but this is the first we've seen of it, and it doesn't appear that many others have discussed it either.