For me it depends on the situation. If its a buffet and all the staff are doing is collecting the dirty dishes, then I probably won't tip.
When I travel in the USA, I generally receive complementary breakfast at places like Hilton Garden Inn due to my HHonours Diamond status. This comes in the form of vouchers for each day.
If the wait-staff are service me coffee, taking an order etc, then I will tip. Usually for basic coffee pouring etc I will just leave cash on the table when I leave. I look at what the cost of the breakfast would have been if I was paying and leave what would be considered a normal tip for that amount, say 15% rounded to the next dollar. So that generally means one or two dollars for most places I stay.
At places where a hot breakfast is cooked to order, such as Embassy Suites, I will generally least a dollar bill for the cook on the counter when collecting my cooked meal. In that situation I do not leave anything on the table when leaving. At Embassy Suites the rest of the breakfast service, including coffee, is self-serve so its only the cook that gets the tip. If I choose to just have toast, juice and coffee, then no tip is left.
When travelling in the USA I receive a generous per-diem allowance that is designed to cover all meal costs. So if I receive a complementary breakfast by virtue of my hotel chain status I am happy to depart with one or two "dead presidents". If breakfast is included in the room rate (and I don't mean just donuts and coffee as you might get at a Best Western or Hampton Inn, but a real "Bed and Breakfast" room rate) than my per diem is reduced by an amount deemed to be for breakfast. In that case I do not tip as I see it as being included in the room rate.