When looking at the "big picture" of destinations, Wikipedia is amazing! You can see which airlines fly where out of which airports, and a list of an airline's destinations. Just keep in mind, sometimes direct (but not nonstop) flights will show up... for example, there's a UA flight that goes GUM-HNL-ORD, and the wiki page shows GUM as a UA destination from ORD. Also, sometimes a list is somewhat divided between a mainline carrier and their regional partners. Usually one has a link to the other if that's an issue, and overseas flights are almost always mainline anyway.

Here, this should help you out!
Ryanair destinations - Wikipedia, the free encyclopedia
Delta Air Lines destinations - Wikipedia, the free encyclopedia

Unfortunately, the Delta one isn't as organized as United's, which goes by continent. However, if you group by country, you'll be a little more organized at least.