Fretello - Duolingo for learning how to play guitar
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Justin Guitar - Over 750 completely free guitar lessons!
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Uberchord Guitar - Get better on your guitar. Fast.
Qiqqa - Qiqqa is a free research and reference management software. It can be used in many organizational projects from the academic to the personal to the business endeavor. Read more about Qiqqa.