Here are a couple of possibilities. I've only listed basic ingredients. Add cayenne, parsley, black pepper, etc., as desired. Neither will give you exactly the same taste and texture as a classic hollandaise, but both are acceptable substitutes.
Yogurt Hollandaise Sauce (Low Fat)
1 cup of plain yogurt
3 egg yolks
1 tsp lemon juice
1/2 tsp salt
In a heavy sauce pan, whisk all ingredients together except lemon juice. Cook over low heat (or in a double boiler), stirring constantly until thickened (about 15 minutes). As you cook, the sauce will actually become thinner, then after about 10 minutes it will begin to thicken again. Remove from heat, stir in lemon juice and any other desired seasonings. Serve warm.
Buttermilk Hollandaise Sauce (Low Fat)
3/4 cup buttermilk (or 3/4 cup skim milk plus 1 Tbsp fresh lemon juice or vinegar)
1 Tbsp cornstarch
1/2 tsp salt
1 large whole egg, lightly beaten (or 2 egg yolks)
1 Tbsp fresh lemon juice
Whisk buttermilk, corn starch, salt and egg together in a heavy sauce pan or double boiler until smooth. Cook over low heat, whisking until it's thick. Remove from heat. Stir in lemon juice and any other desired seasonings. Serve warm.