Atkins used to recommend sucralose sweeteners - that's Splenda.
Most tablet sweeteners do have some carb - Splenda tablets have 76.9 grams of carb per 100 grams, Canderel tablets have 87.4 per 100 grams, Hermesetas Stevia tablets are actually zero carb for 100 grams.
Be careful of granulated sweeteners, the stuff they add to bulk it out makes it have nearly as many carbs as sugar. Granulated Splenda for instance has 97.7 grams so is not a lot different from sugar.... I can tell you I was not pleased when I discovered that! Tablet sweeteners are better if you are trying to cut carbs.
The different sorts do taste a bit different, I like the Hermesetas Stevia myself but that's just me.