iTALC is a use- and powerful didactical tool for teachers. It lets you view and control other computers in your network in several ways. It supports Linux and Windows XP (Vista/7 soon) and it even can be used transparently in mixed environments!
In contrast to widely used commercial equivalent software, iTALC is free! This means you do not have to pay for expensive licenses or things like that. Furthermore the source-code is freely available and you're free in changing the software to fit your needs as long as you respect the terms of iTALC's license (GPL). Freedom in two ways!