The guys at Panic just released an update to Coda which, among other very useful features, includes a new plugin API.
I’ve hacked together a quick plugin around the AutoHyperlinks framework that will find raw URIs and email addresses and link them. URIs in existing markup is skipped, mostly.
Quality is still a little rough, but it handles simple markup well (and you can always run it on just a selection of text).
I’m releasing the plugin under a 3-clause BSD license. More to come, I hope.
Enjoy.