Umlaut, developed at Johns Hopkins, calls itself an “open source front-end for a link-resolver.” There are examples of connecting this to the Ex Libris SFX Link Resolver via API. Umlaut provides more flexibility on the link resolver interface, and more integration possibilities.
Github page: https://github.com/team-umlaut/umlaut/wiki
Code 4 Lib Umlaut page: https://wiki.code4lib.org/About_Umlaut
View a webinar by the creator of Umlaut: http://www.oclc.org/research/events/2012/08-01.html