Keras
Francois Chollet released a wrapper in which a network is described in a dozen lines and the lower-level complexity is hidden.
Why it matters
Deep learning became accessible without understanding computation graphs, and the number of people who could train a network grew by an order of magnitude.
The first version ran on Theano, TensorFlow support followed, and in 2017 (TensorFlow 1.4) Keras moved into the TensorFlow core as tf.keras. Simplicity cost flexibility, and research groups mostly stayed at the lower level. But for applied work and teaching it became the usual entry point.