The best stuff I've used was given to me by a guy at Warwick services a few days ago. I'd stopped to fill up the washer bottle with water (ideally with screen wash, but at a pinch, anything vaguely wet!) but the tyre pump and water machine was out of action. The guy that had stopped there to pump his tyres up came across to tell me that the machine was out of action and, when he saw me with the bonnet up, came back with a huge plastic bottle containing what looked like urine - bright fluorescent yellow :-) He very kindly filled up my screen wash reservoir with the stuff - he said he could get it in bulk through work, so he could easily spare some.
Despite temperatures of -8 over the past week, the washer jets have worked immediately I start driving, whereas the concentrate that was rated down to -10 that I used before was usually frozen solid and I had to take the pipes off the bonnet each morning to break up the ice crystals and squirt de-icer over everything.