From my limited personal experience - I had great success with ezinearticles.com and associatedcontent.com, both free. (In fact, technically Associatedcontet.com pays you, but it's generally going to be very little.)
I tried isnare - my three articles for them got me, so far, a total of 12 visitors. Now, they're cheap - it works out to $2 an article for them to send your articles out for you - but writing articles takes time, so I didn't find them to be worth it.
That said, it might be worth it for you to experiment with a few paid article directories, but I would start small and see how it goes; I wouldn't throw a bunch of money at it up front.
You would want to submit your article to the top article directories. I found this page that ranks the top ones by Alexa and also Google PR. You can check it out here.
Also Jack Humphrey offered his top 21 article directories on his blog you can see here.
Most article directories are free. The charges are normally for article submission for distribution.
Hope this helps.