I drank many a tamarind soda during hot childhood summers in Mexico, and created this much-less-sweet version with lime and agave nectar. The salty-spicy rim offsets these flavors and makes them shine. A tip: it's easier to squeeze limes, and they express more juice, if they're at room temp; also try rolling them on the counter first. Oh, and some tequila could slip in here, too... —gluttonforlife
Combine lime juice, agave nectar and tamarind in a small saucepan. Whisk together over low heat, just to combine; about 2 minutes. Strain into a pitcher and chill.
Stir the salt, sugar and chile powder together to combine well and mound on a small plate.
Rub a juicy piece of lime around the rim of 6 glasses. Dip the rims into the Chile Salt to coat well. Fill the glasses with plenty of ice. Add 1/4 cup of the syrup to each glass and top off with seltzer. Stir well and garnish with a slice of lime.