A couple of months back I posted a brief article about Cicero’s project to get an ENC28J60, low-cost ethernet module working with the ESP8266. There was a something of a hiatus with the project, with no updates appearing for a while (due to “real life”™) getting in the way, but Cicero (Mark) has been back at work, putting up the project code on GitHub, along with hardware connection details.
I note that the current implementation only runs the ethernet hardware and not both WiFi and ethernet at the same time, so there might be some contention for resources there, but I don’t have an ENC28J60 module to test with. If you do, Mark is looking for feedback and submissions for the project, so have at it!
On a related note, has anyone tried using a Wiznet 5200 or 5500 module (such as the Wiz-820io) for a wired connection? They’re more expensive, and perhaps not quite as versatile, but there should be considerably less overhead on the ESP8266. Let us know in the comments if you’ve built (or seen) such a beastie.