In the last year I started experimenting with Reaper's video functionality and
I tried to put together a cool list of presets for the Video Processor plugin.
Some of them have been blatantly stolen from the Reaper forum, some others
are modified presets or new ones made from scratch.
Feel free to change and/or improve them as you wish.

I also want to clarify that I will often make substantial changes to my presets until I find, at least for my needs, a good and clear workflow.

Presets list (v0.7.2 - see changelogs here):

- Track opacity/zoom/pan+
Same as the default one but with a primitive mask (only without rotation),
  custom shadow support and more.

- Proportional grid
Shows a grid of guidelines.
- Blend
Blends two tracks with add, subtract or multipy mode.
- Freeze frame
- Multi flip
Flips the video horizontally, vertically or both at the same time.

- Rectangle
Generates a colored rectangle.
- Ellipse
Generates a colored ellipse.
- Gradient
Generates a colored gradient.
- Strobo
Generates a stroboscopic effect.
- Checkerboard
Generates a colored checkerboard pattern.
- Text overlay+
Same as the default one but with colored shadow, the ability to
  increase the space between letters and change the font style.

- Invert colors
- Repetile
Repeats the video in multiple directions.
  Similar to AE's CC Repetile.

- Kaleido (source)

- Tiler
Fills the screen with multiple copies of the video.
  Similar to AE's CC Tiler.

- Basic mirror
- Optical Distortion
Simulates the distortion of a lens.
- Noise
Fills the screen with random colored pixels.
- Wave morph (source)
- Motion blur (source)

How to install/update:
1) Extract the downloaded compressed folder somewhere
2) Open a new Reaper project and add a new track with a Video processor
3) Press on the plus button, select "Import preset library" and choose the "preset_library.rpl file"

Download preset library

If some of the presets appear in the wrong category you can delete the "builtin_video_processor.ini" file in the "C:/Users/USERNAME/AppData/Roaming/REAPER/presets" folder and then reimport the library.
Remember to save your personal presets or they will be deleted too!