I discovered cocktail fruit a couple years ago. Except for the color, they look like a normal grapefruit, and are sometimes called cocktail grapefruit, but they're a cross between a pomello and a mandarin. The skin color can range from normal grapefruit yellow to a deep kelly green (and the skin color has nothing to do with ripeness. They're ready to eat, no matter what color the skin is. I like the deep green ones, just because they're so pretty).
They do have a lot of seeds, but the flavor is so yummy. Sort of like lemonade with just a hint of orange or tangerine. It doesn't have any of the bitterness of grapefruit.
I've noticed that they're starting to show up in more grocery stores, but locally they're usually cheapest and most available in asian markets.
And best of all, they're only about 120 calories for a very large fruit.
You can cut them in half and eat them with a spoon like a grapefruit (but they don't need sugar) but I eat them like an orange, seperating each section. I sometimes peel the membrane off of each section, because it can be a bit tough, but usually I just pop the whole section in my mouth.
The season is November through February.