Here is a compilation of tools created to use with Reaper.
The free loop tool
The free loop tool is a python script that helps you realigning freely played (meaning without a given tempo or click) loops to the grid. It is meant for beat oriented production techniques where the beat of the song is the starting point. With this tool you can simply record a beat, loop it with grid and snapping turned off, select the recorded event and run the loop script. The script will then try to find out how many measures you played, cut out the loop for you, set the song tempo and align the loop to the grid.
Global MIDI transposition tools
This is a set of js effects that can help you to transpose whole songs.