For large organisations and consortia who need the proxy service to present different IP addresses for different parts of their account structure we have an option available where the OpenAthens Managed Proxy Service can use a locally hosted forward proxy to perform the final hop to the publisher site.
OpenAthens still does the complicated parts (selecting the relevant configurations, permissions, redirector support and re-writing), but the final hop to the resource can come from an IP address of your own - i.e. one that vendors already have registered for you. In a sense you are bringing your own IP address to the managed proxy service.
There are three options documented.
For the large organisation or consortia with a need for different IP addresses for different organisational units:
For the smaller organisations who want the own IP element:
Squid is a widely used proxy available on most platforms under the GNU GPL (i.e. it's popular and free). Link: http://www.squid-cache.org/