Nuun is a product I've used a ton of myself, and I've recommended to at least a half dozen friends over the years. I learned about it myself via word of mouth -- my good friend Stef Samuel, who has done Ironman Triathlons all over the world, swears by the stuff.
Essentially, Nuun is an effervescent tablet that you throw into your drinking water bottle and provides both flavor and electrolytes to improve replacement of the water and salts you lose when exercising. It's like Gatorade without the sugar, and without all of the environmental costs of a disposable plastic bottle, and delivering a bottle of liquid. There's also a major portability benefit - a tube of 12 tabs (good for a dozen 16-oz bottles of drinks) is about the size of a roll of life-savers and weighs a few ounces. You can throw that and a drinking water bottle in your bag and be set for the next month's worth of workouts. (And because there's no sugar, not only is cleanup easy, but if you're exercising to get in shape, you're not ingesting calories that will undo all the work you just did.)
One caveat: some folks may find they need some calories in their sports drinks. I've found that I prefer an alternative to nuun when going to spin classes, because I like to get a little bit of sugar from whatever I'm drinking. In general, though, if you're working out for under an hour, you don't need any calories from your sports drink. I could go with just plain water, but subjectively I feel like I absorb nuun-treated tap water better than plain water, with less of that bloated feeling that can happen when you drink 24-32 oz of water while exercising hard.
It's not cheap, but it's generally cheaper, more convenient and better for the environment than drinking Gatorade, Powerade or other pre-made drinks. And it's generally better for you, as well.
The only flavor I don't recommend is Kona Cola, but they seem to have stopped making that. It's still in some of their variety packs and should be avoided. The other flavors are all fine - they're not fantastic, but they're light and innocuous. I guess my second least-favorite is the "citrus-fruit". Mostly, the flavor is there to cover any potential saltiness from the electrolytes, I suspect. If you are worried about liking the flavor I recommend finding this at your local running or cycling speciality store and trying just one tube. If you're going to order online, sign up for their mailing list (they send out coupon codes fairly regularly) and order 9 tubes or so at a time, because the shipping cost is significant but fixed. I recommend the mixed fruit flavors pack, which has 3 each of 3 flavors.
