Sometimes the simplest solutions are among the best. Take Guyot Designs' Splashguard Universal, a $3.25 cap designed to fit all wide-mouth water bottles from Nalgene, GSI, and the like.
As the name portends, the SplashGuard's purpose is to prevent that splashing, sloshing effect that comes when trying to drink while on the move.
There's a small opening on one end from which you sip; a larger opening on the other side lets you add water without removing the cap.
Made of food-grade silicone, the little cap slips in flexible and easy.
The silicone rubber won't absorb or retain odors and tastes, and it can be sterilized by boiling.
Weight: 0.5 ounces
Price: $3.25
Company: Guyot Designs, www.guyotdesigns.com