I went to two of the Greek islands Santorini and Ios this June and stayed at two great places. The first is Youth Hostel Anna on Perissa Beach in Santorini. I warn that this is not the cleanest place, the washrooms are a bit dirty, but bring shower shoes and be adventurous and you'll be fine. It's a fun lively place, alking distance to Perissa beach, which is a black sand beach. One of the best nights on my trip was spent with a few people I had met at the hostel having a bonfire on the beach and watching the sun rise. I met great people here.
On Ios, I stayed at Francescos. People either stay here or Far Out Camping and I'm so glad I picked this one. The hostel is beautiful you will die when you step on the patio and see the view. It's very clean and welcoming and while it is a 20 min walk to the beach you can take the bus which comes fairly often and you are right in town, so at night you don't have to stumble home for 20 min drunk. The poeple who work there are great. They organize drinking games and take you to their bar, where people stay for a while and then go off to others.
Anyways, can you tell I loved Santorini and Ios!