Sage: Open Source Mathematics Software. Simply because I've been thinking about worksheet interfaces lately and considering what options there are - Sage is not just a math package, but primarily a huge collection of Python modules and a worksheet interface for Python as well. Okay, you use a web browser against a locally running web server, but still, it's actually a nice thing. And meanwhile, version 4.7 with many changes is out. And you can repeat good things. It's definitely cheaper than mathematics and I also like Python much better as a programming language. Since I will probably eventually install my own server to access notebooks from anywhere, here's a blogmark on a guide on how to build your own Sage server with Ubuntu.