The best free tool for keyword research is Google itself, especially with all of the features it uses to help searchers find exactly what they're looking for. Take Google Suggest. Added back in 2008 to speed up searches, it offers SEOs a great research tool when researching keywords for both regular SEO and PPC campaigns.
Google Suggest, as you know, is the feature that drops down a list of suggestions from the search box as soon as you start typing. So for example, if you start typing in “search engine optimization,” once you start typing in the word “search,” the text box might drop down with the suggestions “search engines,” “search tempest,” “search facebook,” and “search all craigslist.” Basically, Google is trying to guess what you're going to type next, and save you the time it takes to hit those keys.
So how can this lead to finding more keywords? A post over at Vertical Measures gives one example. After getting tipped off by a client to Google Suggest as a keyword tool, the author started to enter the phrase “link building service,” which his company targets. He got as far as “link bu” before the search text box dropped down with suggestions. Just below “link building service,” Google suggested “link building strategies.” His firm hadn't considered targeting that phrase before, but it made sense to think about doing so now.
Since we're talking about adding new keywords to target with Google Suggest, it makes sense to think about where Google gets the suggestions it makes. Here's the search engine's explanation of this feature: “As you type, Google Suggest communicates with Google and comes back with the suggestions we show. If you're signed in to your Google Account and have Web History enabled, suggestions are drawn from searches you've done, searches done by users all over the world, sites in our search index, and ads in our advertising network. If you're not signed in to your Google Account, no history-based suggestions are displayed.”
If you want to cast the widest net, you'll need to make sure you're signed out of your Google Account (more on that in a bit). As you can see, though, the suggestions Google gives you come from actual user queries, so they should be very good suggestions. In addition to actual queries, Google taps into ads. This means that marketers will pay money for their ads to show up in association with these queries. The implication, then, is that any term that shows up for Google Suggest while you're typing in a keyword you're targeting, provided it's related to the keyword, might also be good to target.
Tony Soric over at Clickable pointed out another way to use Google Suggest: as a negative keyword tool for PPC campaigns. Say you're a clothing retailer trying to build a PPC campaign around the phrase “little black dress.” You might find that as soon as you type in the phrase “little black,” suggestions include “little black book,” “little black ants,” and “little black grasshoppers” in addition to “little black dress.” If you're doing an AdWords campaign, you may want to add those as negative terms, to make sure that your ad doesn't show up for irrelevant searches.
Soric noted that using this approach can make a lot of sense for branded campaigns. As an example, typing “clickable” into Google gives the user a drop down with a lot of search phrases that have nothing to do with the Clickable website; apparently lots of things are clickable, including world maps, pictures, fairy charms, and even oil. “These are prime examples of negative keywords that should be added to our branded AdWords campaigns,” Soric observed.
Choosing Keywords 
