This is the list of video effects available in Kdenlive and its parameters. They are implemented as part of the MLT Framework.
No effect 
This is the original image with no effects, to allow comparisons.
Box blur 
This effect blur the image.
With this one, you can make the video look as if there was less light -- or more of course.
Here you can get a charcoal pencil draw.
Here you can change the gamma.
This one will help you publishing on very old TV sets ;-)
Inverts the colors of the video, you get something like a argentic negative.
Here you can mirror half of the image, either horizontaly, verticaly, or following a diagonal. The invert option helps on getting the interesting part of the video mirrored.
Allows you to mask a portion of screen with keyframable mosaic effect.
Let's make an old fashioned movie : sepia transforms colors in shades of brown. But with Kdenlive, you can control the color output, so this can be shades of any color.
Allows you to slow down, speed up, reverse play and make stroboscope effect. This effect is available only if Kdenlive was compiled with the latest development version of MLT, taken from its cvs.
Allows you to freeze the clip at, after or before a certain frame. You choose a frame that will be shown for the whole clip, or just a part of it.
This effect is available only since Kdenlive version 0.5.