Zotero is a free tool designed to help you gather, manage and share information about books, articles, web pages and other digital objects that you are using in your research, and to generate citations and bibliographies. Zotero is a Mozilla browser plugin. You can choose to gather your data on your hard drive (if you are working mainly from one computer), or on a flash drive or a network (if you frequently access your data at multiple workstations).
GNU is an open source operating system based on Linux. In addition to offering a freely available operating system, the community maintains a number of open source softwares including many usable in Windows.
Opensource.com provides a list of tools and applications you can use to replace some of the proprietary tools you may have gotten used to being able to use at Georgetown. Check out their opensource alternatives to MATLAB, for example.
The social site reddit is a surprisingly rich place for finding new tools, applications, and other useful free resources. They have subreddits devoted to open source technology, tips and tricks, notifying when apps become free, and many others. As always, be wise about what you decide to use.