Preheat oven to 450 F. Slice bread on the diagonal in 1-inch slices; place on a shallow baking pan in a single layer. Bake until toasted, about 2 minutes on each side. Remove from oven; spread about 1 tablespoon hummus on each slice or press some feta cheese firmly onto each toast surface.
In a bowl, combine tomato, olives, basil, balsamic vinaigrette and garlic until blended. Top bread slices with tomato mixture, dividing evenly. (If desired, for non-vegetarians garnish each slice with a peeled, cooked small shrimp or a sliver of prosciutto.) Serve immediately.
Makes about 20 crostini.