Citrix-Like Server for Linux?

Devil's BSD asks: "My school is planning to add remote desktop access so that people can easily access a school computer from home. However, with the financial situation in our Kentucky being what it is, using Citrix Metaframe for Unix/XP and buying all the software licenses necessary will be extremely hard. And with the state department of education (ironically named KDE) very pro-Microsoft, VNC is out of the question. Is there a free or low cost Citrix-like software suite that can give access to a remote desktop and compress the datastream to be able to work on a 56k modem like Citrix's ICA does?"