Tiny Planet is an interesting effect to be made from a photo or drawing. I saw it for the first time at Wojciech Toman's devlog. Here are some tiny planets made by me recently:
To make a tiny planet, you have to first take a 360 degrees panorama photo of some landscape. For stiching photos into single panoramic one I recommend free application from Microsoft Research called Microsoft Image Composite Editor (ICE). Of course there are many others availble. Then the process involves some manual graphics work and/or smart usage of some filters, where the crucial one is converting image to polar coordinates. In GIMP you can find the appropriate menu command in Filters / Distorts / Polar Coordinates.
The biggest question when making such images appears to be how to fill the inside and the outside of the circle forming surface of the planet. Do you have any ideas better than the ones I used here?