(Note: It seems to be very difficult to find where to download Cusp at present. If you have trouble, use the following Eclipse update site).
Cusp is maintained by Tim Jasko
Project's site is cusp.devjavu.com (seem to be dead)
Source was found at code.google.com/p/cusp
And there is a clone at github: https://github.com/budden/cusp
Article on CUSP at IBM Developerworks: http://www.ibm.com/developerworks/opensource/library/os-eclipse-lispcusp/index.html