I have a steripen. They are great. It also uses the same cr123a battery as my fenix pd20 and pd30 so it cuts down on the spares I need to carry.

Also one of these Aquamira-Frontier-Emergency-Filters in my survival kit. Cheap, light and fast for limited use.
http://www.amazon.com/Aquamira-Frontier-...04&sr=1-109