BibTex is a free referencing tool most commonly used with the LaTex document formatting system. More information can be found on the BibTex site.
A referencing tool specifically designed for the Mac PC and Apple devices. More information can be found here. It is not free but can be purchased for use on your Mac. A web based sharing facility is also available through Papers Online.
Wizdom is similar to Citeulike in that it is free software to sort, cite and share your research. It has been designed by and for researchers at the University of Oxford and is specifically aimed at Science and engineering subjects but can be used by anyone involved in research.
There are direct links to save your records to Wizdom from several databases including Taylor and Francis, IEEE, PubMed, and Science Direct.
More information about Wizdom can be found here.
Paperpile is a web-based reference management tool available to use as a Chrome extension, or as an iOS or Android app.
It's designed to interact with Google Docs and Google Scholar, and also works together with Microsoft Word.
There is a subscription cost for Paperpile. Further information is available at Paperpile.