As the quantity of data is increasing maintaining the data in one place becomes difficult. In order to manage the data of researchers many open source software are available to have all the data in one organised place. These are referred to as online reference or citation management software. By automating the data one can gather, manage, and cite online references and these reference tools facilitates a more efficient research process. There are many reference tools available which are open source. Of which i am discussing two reference tools Zotero and Mendeley.

Zotero is a research tool for managing online references. It is freely available and can be downloaded from the net on any computer running Chrome, Firefox, Netscape etc. The icon of zotero appears on the browsers address bar when downloaded. Clicking that icon prompts Zotero to gather the bibliographic information for the source being viewed and add it to the user’s Zotero library. If a web page includes multiple sources, the application lets the user save some or all of the sources at one time. Through a pane that opens at the bottom of the browser window, users can manage the sources they have gathered, adding notes, keywords, or attachments to any source. Sources in a user’s library can be grouped into collections, which are represented as folders, resulting in a flexible, personal system for organizing references. Individual sources can be included in multiple collections. A search feature lets users locate sources in their Zotero libraries based on keywords found in any of the data fields.

Users can also take a snapshot of an online resource, which stores a local copy of that source, and add highlights or annotations to these snapshots. Alternatively, users can manually enter a source that either is not online or is on a website that does not interact with Zotero. A timeline tool organizes sources by date and lets users highlight specific keywords in the sources displayed across a timeline. Users choose a citation format, such as Chicago, MLA, APA, or others. To add a source from Zotero, a user simply drags that source into an application such as Microsoft Word or Google Docs, and a properly formatted citation is inserted. Zotero also generates a bibliography of all the sources included in a paper.

Mendeley reference manager is a desktop and web program for managing and sharing research papers, discovering research data and collaborating online. It combines Mendeley Desktop, a PDF and reference management application (available for Windows, OS X and Linux) with Mendeley Web, an online social network for researchers. Mendeley is freely available and can be downloaded on any desktop.
Mendeley requires the user to store all basic citation data on its servers—storing copies of documents is at the user's discretion. Upon registration, Mendeley provides the user with 2 GB of free web storage space, which is upgradeable at a cost.

Mendeley reference manager has been designed for undergrads and PhD's, post doc, professors and librarians.

Undergrads and PhD's can search publications, papers and articles, follow trends and inspirational people, and organise their own research, papers, articles and references.

Post doctorate researchers can develop their skills and network through connecting and collaborating with their peers. They access their research wherever they are. Elevate their profile and develop their network.

Professors can connect students to a world of resources, improve research skills and the quality of student submissions. They can connect their research wherever they are and publish their own research and see its impact in the scientific community.

Librarians and institutional buyers improve research and collaboration, can connect their students to millions of papers and people, support their students with dedicated resources and on boarding from Mendeley improve the quality of research conducted at their organisation.

Apart from these two there are many reference management tools available. One can look at the following site to know more about the  best reference management tools that are freely available online.

References:
https://net.educause.edu/ir/library/pdf/ELI7041.pdf
https://www.mendeley.com/home/c/?e=238
https://en.wikipedia.org/wiki/Mendeley