Plastic bottles will work, but it is an overly-broad question...
- Brown PET will definitely work just fine, but I've heard it said that you can't get them too hot. Rinse with cold water. I'd probly buy some Coopers if I could rinse them with hot water. This bit scares me when it comes to sanitizing.
- Clear bottles will skunk your beer if they're not hidden in a VERY dark place.... such as within 2 paper bags and a box, in a basement. Light kills the hops!!
- Water bottles can potentially work, but you'd be best off sticking to the 20oz, 1L, and 2L soda bottles, since those bottles are, indeed, designed for carbonation. Plastic bottles can hold twice as much pressure as glass bottles.
-- I tried half gallon Ocean Spray bottles and they worked OK. They "blew up" in terms of volume, but they didn't explode or leak. I would use these again if it weren't for the bottom "blowing up" too, so it doesn't allow it to sit perfectly flat.
-- After looking at the Ocean Spray bottles, I noticed a 32 oz Gatorade bottle has a much more rigid and "supported" bottom. I think these will work out just fine. I actually saw some off-brand "sport drink" at my local grocery for $70 cents a piece after tax and might go get 10 or so to test out... if and only if their bottoms look to be as rigid as the Gatorade bottles I'd seen.
Only 20 (32 oz) "sport drink" bottles would be needed for 5 gallons of beer. And no need to purchase caps or "cap" at all. Seems like it's worth a shot to me.