Kdenlive offers a rich set of creative effects. Unless specified, effects are parts of the MLT modules and distributed with MLT. Other effects are provided by active projects, including frei0r.
Normal image

Automask
User can select a rectangle in the image which will be pixelated. The effect will attempt to follow the movement, so that even if it moves, the same zone will still be pixelated
Box Blur
Blur with adjustable horizontal / vertical factor
Brightness
Keyframable image brightness
Cartoon (frei0r)
Simplify image colors
Charcoal
Sort of emboss effect
Chroma Hold
Make all colors except one look greyscale
Contrastor (frei0r)
adjustable contrast
Distortor (frei0r)
Distort image
Dust
Add moving dusts to image
Equalizor (frei0r)
Color equalizing
Flippo (frei0r)
Fip image horizontally or vertically
Gamma
Adjust image gamma
Glow (frei0r)
Glow effect
Grain
Add grain to image
Greyscale
Remove colors
Hueshiftor (frei0r)
Change image hue
Invert
Invert colors
Lens Correction (frei0r)
Distort image
Mirror
Mirror image in several directions
Pixelizor (frei0r)
Pixelate image
RGB Parade (frei0r)
Hard to describe...
Saturator (frei0r)
Saturate colors
Scanlinor (frei0r)
Add scanlines
Scratch Lines
Add moving scratch lines over the image
Sepia
Sepia effect
Sobel (frei0r)
(...)
Square Blur (frei0r)
Blur image
Technicolor
Saturate colors
Tehroxxor (frei0r)
Bizarre effect
Threshold
Adjustable threshold, makes image black and white
Vignette
Add nice gradient around image
Hueshiftor (frei0r)
Change image hue
Wave
Distort image with little waves