From the US to...
Europe: Winter, except for the Christmas/NYE period, where it spikes up for expats going home to see family. Hmph. Autumn and early spring aren't bad, either.
Africa/Middle East: Winter, because the best way to do it is to fly to Europe first, then hop on a reasonably priced flight to your final destination - much cheaper that way than booking a trip through a US airline.
Central America: Summer - but note that in the tropics, that's rainy season, so it's not as fun as peak time. I went in early May - the very beginning of rainy season - and had excellent weather and a mostly dry time.. for only $200 from California!
Cuba: About the same as Central America, as it is for much of the Caribbean. Note that summer is also the hurricane season. Also note that you can't fly directly from the US to Cuba because of the travel ban.
South America: The southern hemisphere gets trickier... Our summer is their winter, so you'd think the price would be lower, but because that's when most people in the northern hemisphere take vacation, prices spike then. And, of course, since the weather is great there during our winter, prices spike then, as well. The best thing to do is ride the "shoulder" seasons of early fall and late spring - my friends are going to Brazil and Argentina in November dirt cheap, and I'm going to Australia in the same time frame pretty reasonably as well.
Another thing to keep in mind - even if ticket prices are high on our end, when it's the low-season it other countries, hotel, hostel and restaurant prices can take a dive, so it can even out depending on the length of your trip!