I love the Mexican 7 layer dip, and recently found this low-fat alternative. Ingredients: 1 can black beans (rinsed, drained, and mashed)
1 tsp Cumin

1 cup/250 mL nonfat sour cream

1/2 cup fat-free mayonnaise

1 cup/250 mL shredded fat-free Cheddar cheese

1/2 cup/125 mL thinly sliced green onions

1/4 cup/60 mL sliced black olives

1 cup/250 mL chopped, seeded tomato

1 can (4 oz/113 g) chopped green chiles, drained

Combine the mashed beans and cumin and spread over the bottom of a 9-inch/23-cm pie plate or other serving plate. In a small bowl, combine the sour cream and mayonnaise. Spread mixture over the beans. Sprinkle with cheese, green onions, olives, tomato, and green chiles. Chill until needed. Serve with tortilla chips.

I would probably add a layer of guacamole (a small one) just because it would make it so much better. The fat in avacados is not the bad type anyway, so a small amount wouldn't hurt.