From the outside, the calamansi looks like a tiny lime (actually tinier than the regular lime size), but the inside looks like an orange. It's also called acid orange, and tastes like a mix of lime and orange (hmm).
Mix the calamansi juice with honey and hot water, and you'll get a great 'honey-lemon' juice. It's a good alternative to lemon and lime when you need to make a dish or sauce that requires more liquid with less acidity.
I like to squeeze calamansi in a glass jar and keep them in the refrigerator. I can quickly grab it when I need it for my cooking.