Zotero is a free and open-source citation management tool that is both web and computer-based. While it uses the internet to import and export information, it's major functions are linked to the computer app.
For programming types: join the Zotero community and get involved improving the tool either by reporting bugs, adding to code, creating plugins, or creating new tools.
Requires an app download
Requires the app be open to use the citing while writing function