All of the keyword services try to estimate it. My favorite tool of choice is http://www.wordze.com. G-Man
For Google all I know of is Google Trends. I have one website which is the top search result for a keyword phrase which appears in Trends, that gives me a vague number to apply to the graph. When using Google Trends I compare new graphs to that one (use a comma between keywords) and also to graphs for other keywords such as ton and foot... it's a pretty poor method, so I like to use inventory.overture.com instead which provides keyword data for Yahoo searches. The keyword tool in AdWords does provide a numerical range for keywords..
IMHO Google AdWords Keyword Tool is the best https://adwords.google.com/select/KeywordToolExternal?defaultView=3