I have never used the bitcoinvanitygen service and cannot vouch for it, but the website says it's free. If you try it I'm interested in hearing how good it is.
http://bitcoinvanitygen.com/I'm trying it out. I'm currently on the queue.
Though the limits are pretty low. Only 5 letters vanity and 6 for no case sensitive.
its pretty resource intensive to generate anything higher than that you could just install vanity gen on your computer which is a simple program to use and that would tell you the same thing 5-6 is the sweet spot.