Hakan & Turkan's place was a nice place to crash while exploring Istanbul. It's in a side street off the main road so it's generally quiet at night. Connections to public transport are close and it's only about a 20 min walk to Taksim Sq.
All the amenities you need-washing machine, drying rack, fridge, stove, etc, are there. The bed is comfy too. Their son, Yavuz, is very helpful and gave us his contact info in case we needed anything. Recommended if you're looking to stay in a central area on the European side. (However the ferries to the Asian side are not far away either)