A number of unofficial third-party tools are available for use with Duolingo. Use of these tools is not explicitly endorsed by Duolingo. Caution should be exercised whenever using such tools.
Browser extensions are pieces of software you can install in your web browser to provide additional functionality. Browser extensions available for use with Duolingo are listed at Duolingo Browser Extensions.
Custom Flashcards is a tool developed by Duolingo user montraydavis. The latest information on Custom Flashcards can be found at https://www.duolingo.com/comment/8633527.