Here is a quick tutorial on how to make a (video) transition effect that looks like turning a page, final result can be seen at http://www.youtube.com/watch?v=okDdj5W1EY4 
Supposing you have 2 video clips, FirstClip and SecondClip, respectively red and blue in the demo, my idea for turning the FirstClip over and progressively displaying SecondClip in the space newly avaible is to
1. Wiping out FirstClip with a wipe mask that suggests the revelation of SecondClip
2. Compose an image that figures the back for FirstClip on top, herein called BackCorner
1. Starting point: a projet in 480x270 resolution, with 3 video channels: BackCorner on top (download from attachements if needed), FirstClip in middle, and SecondClip on bottom. (see attachment screenshot: "Page Turn final result")
2. Download & Install the appropriate wipe mask from kdenlive.org (called "page curl wipe", see attachement "Page Turn wipe mask online") through "configuration > dowload wipe mask".
3. Create Wipe effect between FirstClip and SecondClip, applying the mentionned wipe mask (and check "Invert" for attaining the expected effect), see attachement "Page Turn wipe details".
This effect should have the exact duration of the final expected transition.
-- at this stage the project monitor should already give you a preview of the wipe between FirstClip and SecondClip, that reveals SecondClip progressively from top-right corner --
4. Add Composite effect between FirstClip and BackCorner, simultaneously to the former Wipe effect, and tweak the size and location of BackCorner to fit the expected effect, i.e:
- at frame 0 of effect: size close to 0 (4x2 for me), location: top right
- at last frame of effect: full size, location: accross the left border, horizontally aligned.
- at a frame around 1/3 of the effect: full size, location right, horizontally aligned.
If you succeeded the composition definition, the BackCorner should actually go down while growing for 1/3 of the effect, then move horizontally and out of picture to the left for 2/3 of it. see attachement "Page Turn composition details".
5. preview and adapt the position of the pivot frame in the Composite effect if you see some glitch.
-- you're done, easy wasn't it?! --
Thanks for your attention.
Disclaimer & info:
- Sorry for the resolution, that's what I use for web production, feel free to resize the given material to your needs.
- Sorry for the bad english, feel free to correct.
- Sorry for screenshot language & not providing the exact kdenlive menu terms, my install is in french and I have no clue how to get it in english.
- I won't make a video tutorial out of this, due to lack of time. It would be great if you could do it! a credit is always welcome.
- the wipe mask is in attachement as png, for pgm files can't be uploaded here ("Page Turn wipe mask").
- This tutorial has been made & tested with kdenlive 0.7.5, on Kubuntu 9.04, and might work with other versions with no guarantee.
- Feel free to experiment with variants, like the yellow BackCorner instead of the white one.
- Any feedback/improvement/comment is welcome.