Cartoon Images
This is not exactly going to make your image a cartoon but it will enhance lines and darken your image
1. Duplicate your base image
2. On your duplicated layer go to: Filter>>Blur>>Gaussian Blur. I find setting your radius anywhere from 2.0 - 5.0 works best
3. In your drop down menu choose "Multiply"
In my opinion this is a handy effect but you may need to experiment with the radius a bit.
~Chels