Uploadcare vs Resumable.js

Uploadcare doesn't require you to define cached transformations, and handles them quickly for you so you don't have to fiddle with configurations.

Their APIs are open source on github so you can see the source and provide pull requests.

Uploadcare provides a client side widget and API, with well worded documentation that clearly explains how each part of the widget is implemented. The client side API also implements the jQuery promises interface for cleanly defining callbacks and updating with progress.

With the client side API decoupled from the upload interface, you can much more easily create customized UI's that match the look and feel of your site.

Resumable.js splits each file into smaller chunks, this way if the upload of a chunk fails, uploading is retried until the procedure completes. This allows uploads to continue even after a network failure either locally or to the server. It also allows users to pause, resume and even recover uploads without even losing state.

The backend can be found in many languages and for many frameworks.

Resumable is completely free and distributed under the MIT license.