Sweetzfree is liquid sucralose. I've never tried it, but it seems to be very popular among many on this board. It's not sold in stores in the US, and can only be ordered online during certain times of the year - someone will usually post when the "ordering window" is open. It's expensive, but very concentrated, so probably doesn't cost more in actual use than the packets.

If you like flavored coffee, then get out to Marshall's or TJMaxx, and look for sugar-free Davinci Syrups. They use liquid sucralose and are carb-free, and come in many flavors. They are not concentrates though. You can order them online as well, but can usually find them cheaper at the above mentioned stores. There's lots of tasty uses for them.