Deli's often sell this along with their bagels, but it's quite easy to make at home.
I like to use Neufchatel cheese which is similar to cream cheese, but has up to one third less fat. If I happen to have garlic chives on hand, I'll add those for an interesting twist.
8 oz Neufchantel cheese at room temperature
2 tbsp plain yogurt
salt and freshly ground pepper to taste
3-4 green onions, sliced thinly
Mix everything in a small bowl until well combined.