A superb platform for learning bird songs and calls. You can create your own study lists and quizzes and the samples of songs & photos for each species is comprehensive.
For example, here’s the page for the Yellow-bellied Flycatcher track I just listened to. At the top left you can see the list is called: ON Field Prep (I have used this list to prepare for field work since May 2012 so I’m especially loyal to this one). Under the image of the bird, the song and/or call is described in text form (i.e.: “sweet”, “burry”, “musical”, “dry”), and the available photo (left) and song (right) selection has both depth and breadth. With high quality studio headphones, for example AKG 240’s, the variety of sound samples can really be appreciated as each sample provides a slightly different ‘rendition’ of the specific song or call one is looking to learn; these renditions can be explained by certain factors such as vegetation density at the recording site, quality of the recording equipment, distance from subject, and degree of background birdsong.