My fitness pal? It's free and relatively simple to use. You can set a specific calorie allowance per day to eat the amount you would need to lose, maintain or gain depending on your goals. You can also set your own number of calories if you want.
It's good as well to give you an overview of the nutrients in your food so you can see how much protein/fat/carbs you're consuming each day, and see if you've eating too much of one thing or not enough of the other.
The barcode scanner also makes it so easy as, say if you have a packet of crisps, you can scan the barcode and it will come up with the exact thing you've eaten and you can change your portion size.
I've only ever used MFP so have no experience of any other calorie counting apps but there are others out there that I've seen a few posters recommend