Warning! Some information on this page is older than 5 years now. I keep it for reference, but it probably doesn't reflect my current knowledge and beliefs.
Mon 28 Jul2014
There is an interesting photographic effect called Tilt-Shift. It makes a photo of city panorama looking like a miniature due to small depth of field. Wikipedia says it can be obtained optically with some advanced techniques, but it can also be approximated with postprocessing.
Yesterday I visited St. Dominic's Fair in Gdańsk, where I had an opportunity to enter a Ferris wheel and take a photo of my city from some height. Here is my experiment with tilt-shift. I've made it GIMP.
To do it, top and bottom of the photo needs to be blurred. But an out of focus photo is not the same thing as standard Gaussian blur. That's why a special kind of blur is needed. There is a GIMP plugin for it: Focus Blur (Windows binary can be found HERE). In Photoshop, the effect is available as Lens Blur.
Image needs to be blurred more the closer a pixel is to the top or bottom edge of the image. But I have no idea how to do blur (or any other effect) with intensity varying over image location, so here is the trick: We can use only two layers - normal and heavily blurred - and blend between them using layer mask.
So to add tilt-shift effect to your photo using GIMP:
Choose a photo of a city panorama. Open it in GIMP.
Use Crop Tool to crop it so no horizon line or sky is visible, only ground.
In Layers panel, click appropriate button to duplicate layer.
Make sure the top layer ("copy") is selected. Select Filters > Blur > Focus Blur. Change radius to some higher value (I used something around 10-16 for image in Full HD resolution). Confirm with OK.
Now right-click on the top layer and select Add Layer Mask. Mask is created for this layer and selected so now drawing will change layer transparency instead of color.
Select Blend Tool (the one for drawing gradients).
In Tool Options panel, select gradient "FG to BG (RGB)" and change Shape to Bi-linear. Make sure current colors are: foreground = black, background = white.
Draw the gradient by pressing left mouse button in the center of the image, dragging cursor to the top or bottom (hold Ctrl to draw perfect vertical line) and releasing it there.
If you've done everything right, you should now already have blending between layers applied so that top and bottom of the image looks like out of focus. Now you can:
Flatten image by right-clicking on top layer and selecting Merge Down.
Enhance colors. You can, for example, increase saturation and contrast.