I'm back from Iceland, y'all!
Amazing trip, and I'm excited to share; but since I've missed work for a week, I have a lot to catch up on before I have a smidgen of free time. I'll check back in sometime later this week to write a super long post on my adventures but in the meantime, hello again, Network!
I found folks to share a car with on Couchsurfing. You can aslo try the lonely planet forums, as many people post there too.
I spent 10 days with 3 strangers and it was great. We met via CS and met for the first time the day we rented the car. Was epic! So I suggest finding people and booking the car before you go, otherwise you might not get a car OR co-pilots!
Here is Matt's overview, which is a good starting point.
Also, here is a 7 day itinerary, here is a list of tips for a road trip in Iceland , annnnnnd here are some great resources to check out.