Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Simplenote - The simplest way to keep notes. Light, clean, and free. Simplenote is now available for iOS, Android, Mac, and the web.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Laverna - Laverna is a JavaScript note taking application with Markdown editor and encryption support.
EndNote - Accelerate Your Research. Save time, stay organized, collaborate with colleagues and get published with EndNote 20.