+1 on that, and i would prefer http://github.com
I know that http://snippets.dzone.com/ is supposed to host some
treetop grammars, but i never find an easy way to browse them all,
also, there is really *no* benefit compared to github.
A github repo, maybe called treetop-grammars or something, would be
really helpful, but would need some sort of gatekeeper. A different
(or additional) approach, would be to establish a naming convention
for the grammars, something like
maybe, so that it's easily searchable on github. Datamapper or Merb
for example go this way with their plugins. There's no need for a
central gatekeeper and it's still easy to find/fork treetop grammars.
The naming convention should be spoken out loud though, so that almost
everyone will see a reference to it.