Gone - An ephemeral to-do list

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.

Check - Check is a unit testing framework for C, which runs on environments such as GNU/Linux, GNU/Hurd, BSD, and Mac OSX. 

Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.

Things - Things is an easy to use task manager.