here's a quick rundown on how to make one:
Mario Batali - Ricotta Frittata
he stresses simplicity, which is fine, though I tend to pack mine full of veggies. Also, I don't have an oven-friendly pan yet, so I make mine entirely on the stovetop. So instead of throwing it in the oven, I just cover it with a lid and let it cook on a low-medium heat. It does turn out best when finished in the oven, but not entirely necessary.
Anyway, how I prepared mine tonight:
start by sauteeing onions and red peppers. Later, add broccoli, shredded zucchini, spinach, tomatoes, and some pre-cooked chicken. While this mixture cooks down, whisk together 8-10 eggs and some whole milk ricotta cheese.
When the veggies are ready, throw the egg/cheese mixture in, and stir everything together until it's fairly even. Cover the pan and cook at a low-medium heat until everything is nice and firm, like an omelet. If you want, throw some shredded italian cheese on top and let melt.
I find this is a great way to use up vegetables that you wouldn't know what to do with otherwise, or that you don't want to waste.