I love the flexibility of this website as you can use it with classes right across the primary age range. You can select the number ranges you want to use from single intergers to decimal numbers. There is also the facility of entering your own numbers and importantly the site works out the answers too!
I've come across a fantastic interactive whiteboard resource which is great for presenting calculation problems in a motivating way. It's ideal for starter or plenary sessions. The site is called Spinners and it is a random number generator with the added facility of being able to select addition, subtraction, multiplication or division operations.