Google has introduced Google Suggest in 2012. Google Suggest is typically used as a live feature while a user is typing a search phrase into the browser or google website. Google Suggest uses the organic search input of billions of users and try to "guess" that way what a user might be searching for even before he completed entering the query or all the words of a keyword phrase.
Keyword Researcher is designed to search the Autocomplete databases of Google, YouTube, and even That's a lot of keyword sources! As you probably know, if you're building an authority site, or if you would like to become a dominant player in your niche, then starting with a large database of keywords (that your customers are actually typing in) is a powerful marketing tool.
While this one isn’t necessarily a keyword research tool, it will give you valuable insight into how the keywords you’ve been optimizing for so far are actually performing for you. You might realize that you could start ranking for more keywords with more difficult competition, for example, or that only keywords on certain subjects are working for you.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website.  In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC).  I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.