This website is invaluable, i'm not sure if anyone has used the keyword selector tool but it's ideal for discovering a hot market to invest in or for simply identifying the amount of searches each month for a specific set of keywords.

It's on the right hand side of this page, a pop up window will appear, you can then type in your keyword and it will display the total amount of searches for the last month for that particular keyword or phrase.

http://searchmarketing.yahoo.com/rc/srch/