Here you can find a collection of softwares realized by the Scilab tool, these softwares are mainly oriented to the creation of plots; there's no 100% compatibility with Matlab but Scilab is, for sure, a very good free alternative. You can freely download, use or modify all the stuff you'll find here. Anyone who wishes to collaborate and share his experience, is invited to send, his works.
Scilab is a free open source software, download it here.