I maintain Thunar-thumbnailers (
http://goodies.xfce.org/projects/thunar-plugins/thunar-thumbnailers) for the Thunar file manager. Thunar uses a very simple system to register new thumbnailers with the file manager, this is described:
http://thunar.xfce.org/documentation/C/customizing-thunar.html#thumbnailers
A lot of brainstorm ideas mention the speed problems in Nautilus, particularly about thumbnail generation. There are some asking for more formats to be thumbnailed (such as OpenDocument). A lot of this has already been done in Thunar-thumbnailers: look for example at the supported formats on the link I posted.
Also, Nautilus is very very slow at generating thumbnails of JPEG images, because it reads the whole image and scales it down. Thunar just extracts the digital camera's thumbnail from the image and uses that.
An example of this speed difference is (benchmarked on an Intel core 2 duo), per thumbnail:
nautilus took about 2-3 seconds,
thunar did the whole directory faster than I could blink (it you want a number it's less than 1ms per thumbnail).
I think if everyone used and contributed to the same set of thumbnailers, a lot of effort would be saved and quality would improve.
