We use RemoteScan (
RemoteScan® - Use Scanners with Terminal Server and Citrix® ) which is the standard software solution for sharing any kind of document, card or multifunction scanner with other PCs on any kind of network. It has been around for quite a few years, and a search in google for remotescan turns up over 30,000 links and reviews.
They also have a RemoteScan for Terminal services and RemoteScan for Citrix version, which lets people use scanners with any kind of hosted application.