In the most extreme example, see the sweatshops or children harvesting cocoa in Africa.
But even in the 'normal' situation, where workers receive a decent wage, enough for a 'middle class' standard of living, health insurance, and can put savings away for retirement there is still exploitation.
For example, when I do consulting work, my company and clients agree on the value of my labor in terms of an hourly rate. As a wage, I receive roughly 15% of the agreed-upon value of my work. While I would never expect to receive 100% of the value of my labor - after all, some of that $ does have to go to keeping the lights on, rent, maintaining equipment, etc., 15% seems pretty low to me, even though I am fortunate enough to live comfortably.
Every single dollar of profit is simply the unpaid labor of workers. It's not unique to capitalism, either. Any class society that is divided into two classes - the workers and the rulers - is going to be exploitative. One class, the workers, generates the wealth, and the exploiter class expropriates it for themselves.
The exploitative nature of labor in the capitalist system isn't as easy to see as, say, under slavery, because it is hidden by the wage system. It still exists, of course.