If you don't have Photoshop, get GIMP (
GIMP - The GNU Image Manipulation Program)
Then join the Cartographer's Guild -
Cartographers Guild :: Homepage
There's one method I found that just uses the render clouds function. Once you've created a bunch of randomly formed blobs, you can choose which blobs to use, combine, delete, and otherwise work with these things. Once you've done that, then you just need to start deciding what goes where on the map. A sample multi-continent project that I started using this method is attached.