I recently discovered this drink in Prevention magazine. It's a Skinny Margarita and I like it even better!
In a shaker:
1 oz tequila
1/2 oz triple sec
2 Tbsp fresh lime juice (about 1 lime)
1 tsp Agave Nectar
3-5 ice cubes
Shake until really cold.
Pour all contents into a glass (with some ice) adding an equal amount of sparkling seltzer water.
It is light, refreshing, and comes in at about 130 calories (less than half of a regular margarita!).