Written by Olivo Miotto, this extension is hosted on the website of the Institute of System Science at NUS. As the webpage says:

If you're an NUS researcher or student, you're probably familiar with this scenario: you search for a journal paper, you find the link (e.g. from PubMed) to the paper, only to find out that you need a subscription to get the full text. NUS has the subsciption (often) but you still need to log in. The savvy ones know that you can append '.libproxy1.nus.edu.sg' to the journal's host name to do this. This extension does the host name appending for you, so it belongs to the category 'dead simple but really useful'. If you find yourself at a journal site, you can right-click and select 'NUS Library Proxy' from the menu. The page will be reloaded via the NUS Library Proxy, so you can have full text access if NUS subscribes to the journal.