I don't have any experience with renting a car on Alonissos yet, but below is a list of telephone numbers and email addresses of car rental companies on the island.
If you have a good tip for a cheap and reliable car rental on Alonissos island then please let me know. We want to rent a car during our visit (which is probably in 2008, but maybe later) for one or two or maybe even three days, so we can explore the island at our own pase.
It is allways great if you can just make a stop whereven you like and get out to have a look at something. In short: it's different than with a bus. You can mail me through the left side menu of the front page / main page of all the islands, where it says "Mail me".