Mapquest.com is one common source because the Mapquest servers are open. These are not the highest quality aerial images, but they are global and readily available. Many of our examples use Mapquest tiles:
Mapbox.com provides aerial tiles with registration and a monthly fee based on the number of tiles used. These are higher resolution tiles than Mapquest provides.
Bing and Yahoo also have aerial tiles under an arrangement similar to Mapbox.
Other options include: