Keyword Search Volume Tools are useful for getting a baseline of how often a term is searched.  The one I have been using is at WordTracker.com.

http://freekeywords.wordtracker.com/keyword-suggestion-tool.html

Its algorithm is used by collecting data from searches performed at Dogpile.com and Metacrawler.com.  Granted these engines only account for 0.63% of searches across all the search engines, but everyday they do collect about 3.21 million search terms which is good enough for a nice free tool to get you started.

Now even if you wish you had Google's numbers or a large cross market segment you can still get good idea of which terms you should target by comparing similar ones and looking at the numbers. 

I used this tool to look at some Search Engine Marketing for a new site www.atlantaoccasions.com a new site launching to handle special events in my local area.  One of the key areas they want their site to reflect expertise, is weddings. 

Running a quick search at WordTracker, reveals that the term "wedding" is searched for 3,709 times/day.  One of the top reasons to use WT though is that they give you similar terms, and you find out that brides-to-be happen to search the term "wedding dresses" more than twice as much or 8,000 times/day.  So, keying on "wedding dresses" as a term will give them target them toward twice as many searches than just wedding alone.  Then as an extra bonus the "Keyword Difficulty" (which I will blog on later) was also actually lower as well, making this a key term they otherwise might not have known to target.

What tools do you use in determining keywords for your site?