Camelback tablets when I was at REI. They are sugar free and you drop one tablet into a 24 oz. bottle of water. You don't even have to shake it up to get it to fully dissolve. The flavor takes a bit getting used to since it isn't very sweet tasting and a little on the salty side but I liked them a lot for spin class and I think they give me a little more energy. They are kind of pricey - $10 per tube of 12 tablets. I really like the format because the Propels can get a bit messy if you spill them and I was always afraid of a packet bursting in my gym bag. I've only tried the Lemon-Lime but they also come in Berry and Orange flavor.
Update - A reader commented that this has Splenda (sucralose) in it (it's at the end of the ingredient list - sorry) so just wanted to let you know in case you like avoid artificial sweeteners.