You are encouraged to distribute your source files (.sif and so on) so others can learn from your work. Obviously this is only if you have permission to distribute all of it. You can find more videos and images on synfig.com, youtube, flickr, google video, maybe google images and maybe on some blogs or articles about synfig. In particular, all of ullebulle's youtube videos have links to the corresponding .sif files.
An example of what the plant layer can do (note - plant layer is broken in all versions of Synfig prior to svn build 620.)
A synfig version of the "Wind" Windows XP wallpaper. Animated version.
Synfig cat, by Rore. With a heavy use of width-ducks on blines.
Tux, with a Synfig logo. SIF file. (With acknowledgment to Larry Ewing and the Gimp.)